Pregnant women experience sleep disturbances due to the normal physiological changes of pregnancy. Sleep duration in pregnancy varies widely, but typically ranges from 6 to 8 hours depending on the trimester of pregnancy (Facco, Kramer, Ho, Zee, & Grobman, 2010; Mindell, Cook, & Nikolovski, 2015). Nevertheless, many pregnant women wake up frequently during the night due to weight gain, body aches, frequent urination, fetal movements, and uterine contractions (Mindell et al.; Sedov, Cameron, Madigan, & Tomfohr-Madsen, 2018; Sivertsen, Hysing, Dørheim, & Eberhard-Gran, 2015). In a sample of 2,427 women (8-32 weeks gestation), Mindell et al. (2015) reported that 76% of these women experienced poor sleep quality. Poorer sleep quality during pregnancy has been related to lower levels of quality of life, higher levels of depressive symptoms, and adverse pregnancy outcomes (e.g., gestational hypertension, preterm birth) (Blair, Porter, Leblebicioglu, & Christian, 2015; Tsai, Lee, Lin, & Lee, 2016; Warland, Dorrian, Morrison, & O'Brien, 2018).
Vitamin D plays a role in the maintenance of the sleep/wake regulation (Gominak & Stumpf, 2012; McCarty, Chesson, Jain, & Marino, 2014). In animal studies, vitamin D target sites have been documented in the neurons in the midbrain, basal forebrain, and hypothalamic periventricular region areas that influence the sleep cycle (Gominak & Stumpf). The brainstem and periventricular areas of the brain are responsible for the retinal projections of light cues involved in the sleep/wake cycle of the circadian clock (Gominak & Stumpf). Thus, vitamin D deficiency may be linked to sleep disturbances (McCarty et al.).
Vitamin D deficiency has different definitions based on the recommending organization. The Institute of Medicine defines vitamin D deficiency as serum 25(OH)D < 20 ng/mL; however, the Endocrine Society defines vitamin D deficiency as <20 ng/mL, but also defines vitamin D insufficiency as 20 to 29 ng/mL; thereby, defining vitamin D sufficiency as ≥30 ng/mL (Del Valle, Yaktine, Taylor, & Ross, 2011; Holick et al., 2011). These discrepancies are dependent on opinions on whether vitamin D has impact on bone health only or if vitamin D also has metabolic and immune function beyond bone health.
Vitamin D deficiency has been related to poor sleep quality among nonpregnant and pregnant participants (Çakir et al., 2015; Cheng et al., 2017; McCarty, Reddy, Keigley, Kim, & Marino, 2012). A recent meta-analysis, which included nine studies, two of which included pregnant women, reported that participants with vitamin D deficiency had increased odds of poor sleep quality (Odds Ratio [OR 1.59, 95% CI:1.31, 1.72]), short sleep duration (OR 1.74, 95% CI:1.30, 2.72), and sleepiness (OR 1.36, 95% CI:1.12, 1.65) (Gao et al., 2018). However, the definitions of vitamin D deficiency varied among the studies based on serum 25(OH)D levels: <10 ng/mL (n = 1), <20 ng/mL (n = 5), and <30 ng/mL (n = 3) (Gao et al.). The two studies included in Gao et al.'s (2018) meta-analysis included pregnant women from an Asian and Turkish population. Among a sample of 890 Chinese, Malay, and Indian women at 26- to 28 weeks gestation of whom 13.4% had vitamin D deficiency (i.e., serum 25(OH)D < 20 ng/mL), Cheng et al. (2017) found that women with serum 25(OH)D < 20 ng/mL were three times more likely to experience poorer sleep quality compared with women with serum 25(OH)D ≥ 20 ng/mL after controlling for prepregnancy body mass index (BMI), ethnicity, age, education, household income, night-shift work status, physical activity, and parity. However, Gunduz et al. (2016) did not find a relationship between vitamin D deficiency and sleep quality among a sample of 91 pregnant women in Turkey at 36 weeks gestation (Gunduz et al., 2016). Therefore, research is limited on the association between vitamin D deficiency and sleep quality among African American and Hispanic pregnant women.
Minority women are at higher risk for vitamin D deficiency and more likely to experience poor sleep quality compared with non-Hispanic White women (Christian, Carroll, Porter, & Hall, 2019; Ginde, Sullivan, Mansbach, & Camargo, 2010; Liu, Baylin, & Levy, 2018). People with darker skin pigmentation are at greater risk for vitamin D deficiency due to increased melanin pigmentation, which blocks ultraviolet radiation and interferes with adequate absorption of vitamin D through the skin (Hossein-Nezhad & Holick, 2013). The purpose of this study was to examine the relationship between serum 25(OH)D levels and sleep quality among African American and Hispanic pregnant women. A secondary aim of the study was to examine if race moderated the relationship between serum 25(OH)D levels and sleep quality among this sample of pregnant women. The literature has highlighted several key variables that influence sleep quality and vitamin D status including BMI, race, gestational age, and maternal age, as such we controlled for these variables in our model (Christian et al., 2019; Liu et al.; Sivertsen et al., 2015).
Design and Sample
We used a cross-sectional design to examine the relationship between serum vitamin 25(OH)D levels and sleep quality among minority pregnant women. An a priori power analysis was conducted using G*Power 3.1.9 to determine the minimum sample size required to find statistical significance using multiple regression analysis with five predictors. With a desired level of power set at .80, an alpha (α) level at .05, and a moderate effect size of .15 (f2), it was determined that a minimum of 92 participants would be required to ensure adequate power (Cohen, 1988). Pregnant women were recruited from a federally qualified health center (FQHC) from a Midwest metropolitan area. Women were included in the study if they were African American or Hispanic, were at least 18 years of age, had low-risk singleton pregnancies, and were between 24 and 32 weeks in gestation. Women were excluded if they had pre-gestational diabetes, chronic hypertension, autoimmune disorders, or mental health diagnoses.
Variables and Instruments
Maternal characteristics. Maternal characteristics (e.g., age, race/ethnicity, marital status, employment, income) were collected by self-report.
Pittsburgh Sleep Quality Index (PSQI). The PSQI is a 19-item (e.g., How long (in minutes) has it usually take you to fall asleep?; How many hours of actual sleep did you get at night?; How often have you had trouble sleeping because you cannot get to sleep within 30 minutes?) instrument that measures sleep quality symptoms during the prior month on a 4-point scale (0 = not during the past month; 3 = three or more times per week). The tool has seven component scores that include subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, use of sleeping medication, and daytime function. The global score can range from 0 to 21. A global PSQI score > 5 indicates a “poor sleeper.” The tool has been reported to be reliable and valid among nonpregnant populations (Buysse, Reynolds, Monk, Berman, & Kupfer, 1989). The Spanish version of the PSQI has been validated among low-income Peruvian pregnant women at ≤ 16 weeks gestation (Zhong, Gelaye, Sánchez, & Williams, 2015). The PSQI was reliable in the current study (Cronbach's α was 0.71 for the English version and 0.74 for the Spanish version). The participants were recruited from 24 to 32 weeks gestation, thus, their subjective sleep quality was assessed as early as 20 weeks gestation because the instrument asks participants about their sleep quality in the past month if they were at 24 weeks gestation.
Serum 25(OH)D. The serum 25(OH)D levels were measured by radioimmunoassay (RIA) by Quest Diagnostics using Diasorin nonchromatographic methodology (Quest Diagnostics, 2015), the most common clinical methodology used to measure total 25(OH)D (Hollis, 2010). The minimum detection limit of the RIA is <5 ng/mL. Both intra-assay coefficient of variation (CV) and inter-assay CV are 5.1% (Farrell et al., 2012).
The study was approved by the Institutional Review Board at a large Midwestern University. The FQHC where women were recruited was located in Chicago, IL, which is at a northern latitude. Individuals living in the northern latitude are at higher risk for vitamin D deficiency (Hossein-Nezhad & Holick, 2013). Data were collected during the spring season. Seasonality is also a risk factor for vitamin D deficiency with winter and spring months having less sun exposure compared with summer and fall months (Hossein-Nezhad & Holick). The principal investigator approached women for participation before or after prenatal visits to discuss the study. Women who were interested completed an informed consent process prior to data collection. Women completed questionnaires and had blood drawn by the clinic staff into a 2-mL sterile tube. The blood samples were processed the same day by Quest Diagnostics for serum 25 (OH)D levels analysis according to the manufacturer's specification. Women received $20 cash for their participation. The results of the serum 25(OH)D levels were provided to the clinicians who reviewed the test results with their patients. Funding for the honorarium and laboratory tests were paid for by the principal investigator's research funds.
Data were entered and analyzed into SPSS 25. Descriptive statistics (mean, standard deviation, frequency) were used to describe the sample characteristics. Bivariate correlations were used to examine the associations among sample characteristics, serum 25(OH)D levels, and sleep quality. Hierarchical regression analysis was used to examine how serum 25(OH)D levels predicted sleep quality after accounting for variance associated with several demographic covariates (race, maternal age, prepregnancy BMI, gestational age at data collection). Race was coded as 1=African American and 0 = not African American, however the sample was composed of only African Americans and Hispanic pregnant women. Specifically, demographic variables were entered in Step 1 of the regression equation, and serum 25(OH)D levels were entered in Step 2. Moderation regression analysis was used to examine potential interaction between race and serum 25(OH)D in predicting sleep quality (Cohen, Cohen, West, & Aiken, 2003). Serum 25(OH)D levels and race were entered as predictors of sleep quality in Step 1 of the regression equation, and the interaction term (serum 25(OH)D x race) was entered in Step 2 to assess if race influences the relationship between serum 25(OH)D levels and sleep quality. Moderation was determined to be present if the interaction term was statistically significant.
There were 115 women in the study; 62 women (54%) self-identified as African American and 53 women (46%) self-identified as Hispanic. The women had a mean age of 27 years and a mean gestational age at data collection of 27 weeks. The majority of women were married or living with partners (66%), were employed full- or part-time (55%), and had an annual household income of less than $15,000 (50%). Seventy women (60.8%) had vitamin D deficiency (i.e., serum 25(OH)D levels < 20 ng/mL) and 67 women (58.2%) reported PSQI scores > 5 that represent “poor sleeper” (Table 1).
Bivariate Relationships among Variables
African American race was related to younger age, lower serum 25(OH)D levels and higher PSQI scores. Lower serum 25(OH)D levels were related to higher PSQI scores (Table 2).
Predicators of Sleep Quality
Results of Step 1 of the model indicated that the demographic variables explained 9% of variance in sleep quality (F(4, 104) = 2.51, p = .04, R2 = 0.09). Race was a significant predictor of sleep quality. African American women reported worse sleep quality compared with Hispanic women. Results of Step 2, which included serum 25(OH)D levels, indicated that the predictors explained 17% of variance in sleep quality (F(5, 103) = 4.10, p = .002, R2 = 0.17). Serum 25(OH)D levels were significant predictors of sleep quality after controlling for covariates (race, pre-pregnancy BMI, gestational age at data collection, maternal age). Race was no longer a significant predictor of sleep quality (Table 3).
Moderation of Race on Serum 25(OH)D Levels and Sleep Quality
Results of the moderation regression analysis indicated that Step 2 of the model was significant (F(3, 110) = 6.73, p < .001, R2 = 0.16). However, the interaction term was not significant, indicating that moderation was not present. Thus, African American and Hispanic women displayed a similar relationship between serum 25(OH)D levels and sleep quality.
We found that 60.8% of African American and Hispanic pregnant women in our study had vitamin D deficiency (i.e., serum 25(OH)D levels < 20 ng/mL). African American and Hispanic women are more likely to have vitamin D deficiency compared with non-Hispanic White women (Liu et al., 2018). Based on clinical guidelines from the Endocrine Society and the American College of Obstetricians and Gynecologists, pregnant women at risk for vitamin D deficiency should be screened and treated with vitamin D supplementation to increase their serum 25(OH)D levels to sufficiency (American College of Obstetricians and Gynecologists, 2011; Holick et al., 2011). Clinicians should assess serum 25(OH)D levels and provide vitamin D supplementation for pregnant women who are at risk for vitamin D deficiency, particularly minority women.
Women in our study reported poor sleep quality overall. In our study, 58% of women had PSQI scores > 5, representing poor sleep quality. Our findings are consistent with other studies that found poor subjective sleep quality among samples that included minority women at different gestational ages (Blair et al., 2015; Mindell et al., 2015). Sleep disturbances and disorders during pregnancy have been related to poor health and quality of life of the mother and may contribute to adverse birth outcomes (e.g., preterm birth, gestational hypertension) (Chang, Pien, Duntley, & Macones, 2010; Nodine & Matthews, 2013; Okun et al., 2012). Maternal–child nurses should assess for sleep quality among pregnant women and provide resources to improve their sleep. Nurses should reaffirm the importance of sleep hygiene techniques such as avoidance of caffeine-containing foods or drinks in the evenings, keeping the room dark and cool, avoidance of stimulating activity prior to going to sleep, and establishing a bedtime routine conducive to sleeping (Hashmi, Bhatia, Bhatia, & Khawaja, 2016). Maternal–child health nurses are uniquely positioned to recognize risk factors for diagnosing sleep disorders including obstructive sleep apnea, restless leg syndrome, and primary or comorbid insomnia that may need appropriate referrals for diagnosis and treatment (Nodine & Matthews).
Vitamin D deficiency has also been associated with adverse birth outcomes such as increased risk for preterm birth (Amegah, Klevor, & Wagner, 2017). Based on a comprehensive meta-analysis of observational and randomized controlled trials, vitamin D deficiency was associated with increased risk for spontaneous preterm birth (Amegah, Klevor, & Wagner, 2017). There is still some controversy about vitamin D supplementation as an intervention for improving birth outcomes, but the evidence on its association with preterm birth and preeclampsia risk is mounting. Therefore, understanding the relationship between sleep quality and vitamin D deficiency is important because they both are independently associated with adverse birth outcomes.
This study has some limitations. We found that serum 25(OH)D levels predicted sleep quality among women in our sample but acknowledge that there are other factors (e.g., parity, income, education, marital status) that may influence sleep quality that were not included in the model. In our cross-sectional design study, we found that low vitamin D levels were associated with poor sleep quality, which is often disrupted during the third trimester of pregnancy. Clinical randomized studies are needed to evaluate whether vitamin D supplementation, a safe and relatively inexpensive intervention, could improve sleep quality among pregnant minority women.
- Assess sleep quality among pregnant women.
- Recommend sleep hygiene techniques that could improve sleep quality among pregnant women.
- Make appropriate referrals for diagnosis of sleep disorder conditions such as obstructive sleep apnea, insomnia, and restless leg syndrome.
- Assess vitamin D levels in at-risk pregnant women and determine if supplementation is warranted.
- Assess risk for adverse pregnancy outcomes among women who report sleep disturbances because poor sleep quality has been related to gestational hypertension and preterm birth.
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