Intraindividual variability in sleep is related to sleep quality and psychosocial and academic outcomes.

Sleep is important for physical, mental, and cognitive health. Though existing literature has consistently shown that short sleep duration is detrimental to health and functions, relatively little has been learned about another sleep health dimension ── intraindividual variability (IIV) in sleep patterns across days. To assess sleep IIV, researchers typically needed to monitor sleep for a period of time (i.e., usually a week), which is time- and resource-demanding. To overcome this challenge, we developed the Sleep Intraindividual Variability Scale (SIIV) to make measurements of sleep IIV possible through a one-time questionnaire. We administered the SIIV to 925 college students and US residents in three studies. The SIIV has acceptable reliability and validity and it will be a useful tool for future studies on sleep IIV. Moreover, using SIIV and actigraphy data from the three studies, we investigated the associations between sleep IIV and health, psychosocial, and educational learning outcomes. Results consistently show that greater sleep IIV is associated with worse sleep quality, poorer psychosocial outcomes, and poorer academic outcomes. More importantly, individuals with the recommended mean sleep duration also experience some adverse outcomes when they sleep irregularly. The findings suggest that getting the sufficient hours of sleep does not prevent one from experiencing the negative consequences of sleep IIV. Therefore, sleep interventions should be developed and implemented to reduce sleep IIV, especially in those with irregular work/study schedules. Future research should use longitudinal, prospective, and experimental designs to investigate the causal effects of sleep IIV. Theories and conceptual models are also needed to guide future research on sleep IIV