Insomnia, one of the most prevalent sleep disorders, is characterized by dissatisfaction with both sleep quantity and quality. Individuals with insomnia commonly experience difficulty initiating and maintaining sleep, frequent nocturnal awakenings, and early-morning awakenings with an inability to return to sleep, often resulting in daytime fatigue and impaired functioning [1,2]. Chronic insomnia is associated with multiple adverse health outcomes, including reduced quality of life and an increased risk of chronic conditions such as cardiovascular disease, musculoskeletal disorders, diabetes, and cognitive decline, as well as heightened physical and psychological morbidity. In addition, sleep deprivation has been linked to altered immune function [3-7].

Globally, the prevalence of insomnia varies between 10% and 60%, depending on population characteristics and assessment criteria [8-10]. In Malaysia, the prevalence of insomnia is also notable. A community-based survey conducted in 4 urban regions reported that 33.8% of adults experienced insomnia symptoms, while 12.2% had chronic insomnia [11]. Numerous studies examining sex differences in sleep disorders have consistently shown that women are more likely to experience insomnia symptoms than men [12-14]. Despite this high prevalence, insomnia among Malaysian women remains an underresearched area, especially in relation to potential nutritional or physiological determinants.

Vitamin D deficiency (VDD) is another common yet often overlooked health issue in Malaysia. Although the country receives abundant sunlight throughout the year, several studies have reported a high prevalence of VDD among certain populations, particularly women, due to indoor lifestyles, cultural clothing practices, and limited dietary intake of vitamin D. Furthermore, natural food sources rich in vitamin D, such as oily fish, fish liver oil, and wild mushrooms, are not widely available or commonly consumed in many low- and middle-income countries, contributing to this deficiency [15].

Beyond its established role in bone metabolism, vitamin D has been increasingly recognized for its involvement in neurological and endocrine pathways related to sleep regulation and circadian rhythm. It is suggested that vitamin D plays a significant role in serotonin and melatonin regulation [16,17]. The active form of vitamin D, 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), binds to vitamin D receptors (VDRs) that are widely distributed in several brain regions responsible for sleep regulation, including the hypothalamus, prefrontal cortex, and substantia nigra [15,18]. Vitamin D regulates the expression of tryptophan hydroxylase 2 (TPH2), the rate-limiting enzyme in the conversion of tryptophan to serotonin in the brain. Adequate serotonin levels are essential for mood stability, and serotonin acts as a precursor for melatonin synthesis in the pineal gland via N-acetyltransferase and hydroxyindole-O-methyltransferase (HIOMT) enzymatic pathways [17,18]. Consequently, low vitamin D status may lead to impaired serotonin production and disrupted melatonin rhythms, contributing to poor sleep quality and insomnia symptoms [19]. This mechanistic pathway provides a plausible explanation for the observed association between VDD and altered circadian rhythm and supports the rationale for evaluating vitamin D₃ supplementation as a potential therapeutic strategy for insomnia.

Emerging evidence suggests that insufficient vitamin D levels may disrupt circadian rhythms and sleep architecture, contributing to sleep disturbances such as insomnia. A meta-analysis reported that VDD (defined as serum 25-hydroxyvitamin D [25(OH)D] levels <20 ng/mL) was associated with an increased risk of sleep disorders (odds ratio [OR] 1.50, 95% CI 1.31‐1.72) [19]. In addition, women may be particularly susceptible to insomnia associated with VDD. A study involving 459 women revealed a significant relationship between the circadian rhythm of sleep and dietary vitamin D intake, showing that a later sleep acrophase—the time at which the peak of rhythm occurs—was associated with higher dietary vitamin D intake [20]. Despite growing global evidence linking VDD with poor sleep quality, local data examining this relationship among Malaysian women, a population at higher risk for both conditions, remain limited.

Several studies have investigated the effects of vitamin D supplementation on sleep outcomes; however, findings remain inconsistent, largely due to methodological variability. Majid et al [21] reported that individuals aged 20‐50 years with sleep disorders who received 50,000 IU of vitamin D3 weekly for 8 weeks experienced significant improvements in Pittsburgh Sleep Quality Index (PSQI) scores compared with the placebo group (PG; P<.05). Conversely, Mason et al [22] found that daily supplementation of 2000 IU vitamin D₃ in overweight postmenopausal women did not significantly improve sleep quality or quality of life compared to the placebo (P>.05). The differences in study population characteristics, dosage, and intervention duration may account for these inconsistent findings.

Another limitation in the existing studies is the lack of objective sleep measures that provide quantifiable and reliable data on sleep efficiency, latency, and duration. Most studies relied exclusively on subjective self-assessment tools, such as the PSQI or the Insomnia Severity Index (ISI), which may be influenced by recall bias and perceptual differences [15,19]. Additionally, the inclusion of participants with comorbidities (eg, fibromyalgia, depression, or obstructive sleep apnea) and varying vitamin D status complicates the interpretation of outcomes, as these conditions independently affect sleep quality [23-25].

To address these gaps, the present study was designed to incorporate both subjective (validated questionnaires) and objective (actigraphy-based) sleep assessments, focusing on a homogeneous group of Malaysian women with VDD. This approach is intended to yield more robust evidence on the effect of vitamin D supplementation in improving insomnia outcomes within a clearly defined population. By integrating subjective and objective sleep measures, this study aims to offer contextually relevant insights into the potential role of vitamin D₃ supplementation as a nonpharmacological strategy for improving sleep health among Malaysian women with VDD.

This study aims to evaluate the effectiveness of weekly vitamin D₃ supplementation (50,000 IU) in improving insomnia symptoms among women with VDD. The primary objective is to determine the effect of vitamin D₃ supplementation on insomnia-related parameters, including insomnia severity assessed using the ISI, sleep quality measured by the PSQI, and sleep efficiency, which is defined as the ratio of total sleep time to time spent in bed and is objectively assessed using actigraphy.

The secondary objectives are to evaluate the broader effects of vitamin D₃ supplementation on daytime sleepiness, measured using the Epworth Sleepiness Scale (ESS), and on functional outcomes and sleep-related quality of life, assessed using the Functional Outcomes of Sleep Questionnaire (FOSQ-10). Sleep efficiency is considered an important indicator of restorative sleep quality, while functional outcomes reflect the impact of sleep disturbances on daily functioning, productivity, vigilance, and social engagement.

We hypothesize that vitamin D₃ supplementation will significantly improve insomnia symptoms, as indicated by enhanced sleep efficiency and reduced ISI and PSQI scores, compared with placebo. We further hypothesize that vitamin D₃ supplementation will improve daytime alertness and functional quality of life compared with baseline and the PG.

This study is designed as a double-blind, parallel-group, randomized, placebo-controlled clinical trial to evaluate the effectiveness of vitamin D₃ supplementation (50,000 IU) for insomnia symptoms among women with VDD. The study protocol serial number is VDSI01, and the trial is registered with the International Standard Randomized Controlled Trial Number (ISRCTN) registry (ISRCTN69678101). The trial will be conducted in community settings in Ipoh, Perak, Malaysia, and will follow the SPIRIT (Standard Protocol Items: Recommendations for Interventional Trials) 2025 (Checklist 1) guidelines for clinical trial protocols [26].

Women aged 19‐50 years who reside in Ipoh, Perak, Malaysia, will be screened for eligibility. Participants will be eligible for inclusion if they have a serum 25(OH)D level <20 ng/mL (50 nmol/L) and report insomnia symptoms, defined as a PSQI score ≥5, are not currently taking vitamin D supplements or supplements containing vitamin D in the past 3 months, and are nonsmokers and nonalcohol users.

Participants will be excluded if they are pregnant or lactating, share a sleeping environment with an infant younger than 2 years, or have chronic medical conditions such as diabetes mellitus, cardiovascular diseases, thyroid disorders, osteoporosis, malignancy, malabsorption syndromes, or psychiatric disorders.

Individuals taking medications that could interfere with sleep outcomes or vitamin D metabolism will also be excluded. These include corticosteroids, anticonvulsants, cytotoxic agents, antiretroviral drugs, narcotics, antidepressants, and antipsychotics. Participants using antihistamines, particularly first-generation agents (eg, diphenhydramine and chlorpheniramine), will be excluded due to their sedative and anticholinergic properties, which may alter sleep latency, total sleep time, and sleep efficiency. These pharmacological effects could confound both subjective and objective sleep assessments.

Eligible participants were randomly assigned in a 1:1 ratio to either the intervention group (IG) or the PG using a block randomization method (block size =4) generated by a computer-based randomization tool. Block randomization was used to ensure a balanced allocation of participants to each group, particularly given the anticipated recruitment rate. A block size of 4 was selected based on the 2 intervention arms (vitamin D₃ and placebo).

Allocation concealment will be ensured using sequentially numbered, opaque, sealed envelopes (SNOSE) prepared by an independent third party (an appointed pharmacist) who is not involved in participant recruitment or outcome assessment. Each envelope will contain the group assignment (labelled as group A or B) according to the computer-generated randomization sequence and will be opened only after participant enrollment. The allocation list will be securely maintained by the third party in a locked cabinet, and investigators will not have access to the sequence throughout the study period.

To maintain blinding, the intervention and placebo will be labelled as group A or B, and the identity of each group (IG or PG) will remain concealed until completion of data analysis. Both participants and investigators will remain blinded to group assignments throughout the study period. Participants will not be informed of their postintervention serum 25(OH)D results until completion of the study to minimize the risk of unblinding due to perceived treatment effects. Researchers will have access to laboratory results for outcome assessment; however, group allocation will remain concealed throughout the study. To monitor blinding integrity, any instances of suspected unblinding—such as those arising from perceived treatment effects or adverse events—will be documented during the study and considered during data interpretation.

A detailed participant flow diagram (Figure 1) illustrates the processes of recruitment, screening, randomization, follow-up, and analysis and is consistent with the CONSORT (Consolidated Standards of Reporting Trials) 2010 guidelines.

The intervention is initiated after completion of the 7-day baseline actigraphy recording and continued for 9 weeks. Participants in the IG will receive vitamin D₃ (cholecalciferol) capsules at a dose of 50,000 IU weekly for 9 consecutive weeks. The PG will receive capsules containing inert excipients, administered at the same frequency. Participants will be instructed to consume the capsules after breakfast on the same day each week.

Figure 1 displays the details of the study flow. Baseline parameters will be taken at recruitment, and participants will be randomly assigned into 2 arms. Outcome assessments will be conducted at week-0 and week-9. Participants will be instructed to maintain their normal levels of physical activity, keep their usual diet, refrain from taking any additional dietary supplements, and refrain from attempting to lose weight during the study period.

An overview of the study assessments and measurement time points is presented in Table 1.

Assessments will be conducted at baseline (week 0), midintervention (week 5), and postintervention (week 9).

Feasibility will be assessed based on recruitment rate, participant retention, and adherence to supplementation. Acceptability will be evaluated through participant feedback on intervention tolerability and study procedures collected at the end of the intervention period.

Participants will be monitored for adverse events throughout the study period. Any reported adverse events will be documented and reviewed by the study team. Participants experiencing clinically significant adverse effects will be referred for appropriate medical evaluation. Parameters that may indicate the need for study discontinuation will be assessed, including a serum 25(OH)D level higher than 150 ng/mL (375 nmol/l), a serum calcium level higher than 10.3 mg/dL, hypercalciuria, and a glomerular filtration rate less than 50 mL/min/1.73 m² [29]. The participant will receive the necessary treatment accordingly.

The SPSS (version 23; IBM Corp) Statistics for Windows program will be used to analyze all data. Prior to selecting the appropriate SPSS test for analysis, the data will be assessed for normality and homogeneity. Normality will be examined using the Kolmogorov–Smirnov test and a Q-Q plot. Descriptive statistics will be used to analyze categorical variables, such as participant characteristics. Quantitative data will be expressed as mean (SD), while qualitative data will be presented as numbers and percentages. The primary analysis will evaluate the effect of vitamin D supplementation on insomnia symptoms, determined by comparing the change from baseline to postintervention (week 9) between the intervention and PGs. A P value <.05 will be considered significant.

The statistical analyses will be performed as follows:

The trial adhered to the ethical standards of the Declaration of Helsinki. This study protocol has been examined and approved by the Universiti Kuala Lumpur Research Ethics Committee (UREC), with the approval number (reference number: UNKIL REC/2022/0017). Any protocol amendments will be submitted for approval to the Universiti Kuala Lumpur Research Ethics Committee and will be updated in the trial registry prior to implementation.

All participants provided written informed consent before enrollment. All information and data obtained during this study will be kept confidential. Participants will not have access to their personal information or study data. All hard copy data collected from participants will be securely stored and will be accessible only to the researchers. Additionally, all information and softcopy data will be saved on the principal investigator’s password-protected laptop (1 laptop only). The data are stored on a password-protected laptop integrated with secure cloud storage, with access restricted to the investigator’s account only.

Participants’ names will be converted to codes or study IDs (eg, P01 and P02) and recorded on the collection form. The soft copy of the database will be password-protected, with only the principal investigator and coinvestigator having access. After the study is completed, all data, both the hard copy and soft copy files, will be retained for study purposes for 5 years before being permanently discarded from the database. Individual patient records will not be shared with anyone outside the research team.

VDRs and the enzyme 1α-hydroxylase, which converts vitamin D to its active form, 1,25(OH)₂D₃, are expressed in brain regions involved in sleep–wake regulation, cognition, and mood, including the hypothalamus, hippocampus, prefrontal cortex, and substantia nigra [15,30]. Through these pathways, vitamin D₃ modulates the synthesis of serotonin and dopamine, neurotransmitters essential for circadian rhythm stability, emotional regulation, and cognitive performance [17]. Adequate vitamin D3 levels may therefore improve sleep quality and circadian alignment, reducing daytime fatigue and enhancing functional outcomes such as alertness, productivity, and social functioning [31-33]. In addition, vitamin D₃ has anti-inflammatory and neuroprotective properties that may reduce cytokine-mediated neural stress and oxidative burden associated with sleep disturbances and daytime somnolence [34,35]. By promoting neuromuscular efficiency and stabilizing neurotransmission, adequate vitamin D3 levels may further support psychomotor performance and general vitality during waking hours. The influence of vitamin D3 extends beyond sleep regulation to encompass broader neurocognitive and physiological processes that contribute to daytime functioning.

However, VDD is prevalent among individuals seeking sleep medication for insomnia and may contribute to common sleep-related symptoms, including chronic nonspecific pain, poor sleep quality, and impaired daytime functioning [15]. Several studies have suggested that changes in vitamin D metabolism play an important role in the severity and clinical presentation of sleep disorders. Low serum 25(OH)D levels have been independently associated with sleep maintenance difficulties (OR 0.84) [20]. A study by Zhao et al [36] proposed that measuring serum 25(OH)D levels may be useful in predicting pharmacotherapy treatment outcomes in individuals with chronic insomnia. It has been reported that a serum 25(OH)D concentration <20 ng/ml in individuals with chronic insomnia was independently associated with a higher probability of treatment nonresponse at 2 months (OR 11.636, 95% CI 3.966-34.142; P<.001). Evidence also suggests potential benefits of vitamin D3 supplementation for sleep-related outcomes. A study involving 940 healthy adolescent girls who received 9 weeks of vitamin D3 supplementation (50,000 IU vitamin D3/wk) demonstrated a significant improvement in cognitive performance (P<.001) and a reduction in the occurrence of insomnia and daytime sleepiness. The prevalence of insomnia decreased from 15.0% to 11.3% and sleepiness decreased from 15.6% to 14.7%. Additionally, cases with both insomnia and sleepiness decreased from 8.0% to 6.1% (P<.05) [28]. Improved sleep quality resulting from vitamin D sufficiency may restore circadian alignment and reduce daytime fatigue, thereby enhancing functional outcomes such as alertness, productivity, and social engagement. This theoretical framework supports the inclusion of the ESS and the FOSQ-10 in this study to capture the broader impact of sleep improvement on daily activities and quality of life.

However, findings from vitamin D supplementation trials remain inconsistent. One randomized controlled trial failed to demonstrate any significant effect of vitamin D supplementation on various sleep outcomes in a vitamin D-deficient population. Among 189 participants, 92 received a bolus dosage of vitamin D 100,000 IU (2500 mg), followed by a weekly dose of 20,000 IU (500 mg), whereas 97 were given a placebo. The result showed that vitamin D supplementation did not have a significant effect on mean sleep duration, mean daytime sleepiness score, or recovery from inadequate sleep duration, excessive daytime sleepiness, or insomnia after 4 months of treatment [37]. Similarly, Mason et al [22] found that daily supplementation with 2000 IU of vitamin D₃ did not improve sleep quality or quality of life among overweight postmenopausal women. Compared to the placebo, vitamin D did not improve quality of life (P>.05) or sleep (P=.21) in women. These discrepancies may be attributed to differences in study populations, baseline vitamin D status, dosing regimens, supplementation duration, and reliance on subjective sleep measures.

The inconsistent findings from previous studies underscore the need for more randomized controlled trials incorporating clearly defined populations, standardized dosing protocols, and both objective and subjective sleep assessments. This study protocol outlines the design and rationale for a randomized controlled trial investigating the potential benefits of vitamin D3 supplementation in improving insomnia symptoms among Malaysian women with VDD. The study design holds promise for addressing the underexplored association between vitamin D status and sleep quality within this specific population. The decision to focus on Malaysian women with VDD is strategic, given the higher prevalence of VDD and insomnia in this population. The study aims to provide insights into whether addressing VDD through supplementation can lead to improvements in insomnia symptoms, a significant concern for women’s health and well-being.

While the present study offers valuable insights into the effect of vitamin D₃ supplementation on insomnia symptoms, several limitations should be acknowledged. The recruitment of participants from a single geographical location (Ipoh, Perak) and the use of strict inclusion and exclusion criteria may restrict the generalizability of the findings to broader populations of women with insomnia, particularly those with comorbid conditions or from different sociodemographic backgrounds. This decision was intentional to enhance internal validity and minimize confounding factors. It strengthens causal inference by ensuring that the observed effects can be attributed specifically to vitamin D₃ supplementation, rather than to comorbid conditions or medications affecting sleep or vitamin D metabolism. Future studies should consider multicenter recruitment, broader inclusion criteria, and more diverse populations to enhance external validity and generalizability of the findings.

This study also benefits from its placebo-controlled design and the integration of both subjective and objective sleep measures, allowing comprehensive evaluation of insomnia-related outcomes. The findings are expected to contribute meaningful evidence to the growing literature on vitamin D and sleep health and may inform future nonpharmacological interventions for insomnia among individuals with VDD. Additionally, the findings may pave the way for targeted therapies addressing both VDD and sleep problems in this population. The findings of this study may be generalizable to adult women with VDD in similar community settings. However, caution should be exercised when extrapolating results to other populations, including men, older adults, or individuals with different clinical conditions.

This current study builds upon previous research that has demonstrated the positive effects of vitamin D supplementation on insomnia symptoms. This study aims to provide preliminary evidence on the role of vitamin D3 supplementation in improving insomnia symptoms among Malaysian women with VDD. It focuses on a different population and measures more insomnia-related outcomes, both subjectively and objectively, to provide more reliable findings. Ultimately, this randomized controlled trial will contribute valuable evidence supporting the potential benefits of vitamin D supplementation in improving insomnia symptoms among individuals with VDD, especially Malaysian women. The findings are expected to contribute to a better understanding of the relationship between vitamin D status and sleep and to inform future, larger randomized controlled trials. In addition, it will emphasize the importance of considering vitamin D status in the management of sleep disturbances and advocate for further research to optimize interventions for diverse populations.