1. In this study, one night of moderate light exposure (100 lx) during sleep was associated with increased sympathetic nervous system activation, and in turn, increased next-morning insulin resistance, compared to sleep in a dimly lit environment (< 3 lx).
2. The positive correlation between higher sympathovagal balance and insulin levels suggest that sympathetic activation may play a role in the light-induced changes observed in this study.
Evidence Rating Level: 2 (Good)
Previous research implicates nighttime light exposure as a risk factor for cardiometabolic disease. However, the mechanism behind this association is unclear. This study aimed to investigate the effects of acute light exposure during nighttime sleep on glucose homeostasis. Furthermore, this study aimed to further elucidate whether this relationship was due to melatonin suppression, sympathetic nervous system (SNS) activation during sleep, or reduced sleep quality.
Twenty healthy adults aged 18-40 years old, with habitual sleep duration (6.5 – 8.5 hours) and sleep onset (between 9 PM – 1 AM) were included from Chicago, Illinois, United States. Several exclusion criteria were listed in the study, including participants with sleep disorders, neurocognitive disorders, psychiatric disorders, and obesity. Participants were subsequently randomized into a room light condition (n=10, one night in dim light at < 3 lx, followed by one night with room light at 100 lx) or a dim light room condition (n=10, two consecutive nights in dim light < 3 lx). Glucose and insulin levels were measured. Additionally, various elements were also studied, including sleep microstructure and macrostructure, blood pressure, heart rate (HR), and heart rate variability (HRV).
In this study, insulin resistance levels were higher amongst participants in the room light condition; furthermore, these participants also spent significantly more time in stage N2 of sleep and less time in slow-wave and rapid eye movement during sleep. Higher average HR and lower HRV during sleep were also observed in participants randomized to room light, suggestive of increase SNS activation. In addition, lower HRV (ie. increased SNS activation) was correlated with higher insulin resistance. However, this study was limited by its small sample size, thereby limiting generalizability of results. Nonetheless, this study was significant in suggesting that room light exposure during sleep may impair glucose homeostasis through increased SNS activation.
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