Light exposure during sleep impairs cardiometabolic function

Abstract

SignificanceAmbient nighttime light exposure is implicated as a risk factor for adverse health outcomes, including cardiometabolic disease. However, the effects of nighttime light exposure during sleep on cardiometabolic outcomes and the related mechanisms are unclear. This laboratory study shows that, in healthy adults, one night of moderate (100 lx) light exposure during sleep increases nighttime heart rate, decreases heart rate variability (higher sympathovagal balance), and increases next-morning insulin resistance when compared to sleep in a dimly lit (<3 lx) environment. Moreover, a positive relationship between higher sympathovagal balance and insulin levels suggests that sympathetic activation may play a role in the observed light-induced changes in insulin sensitivity.

Keywords: insulin resistance; light; metabolism; sleep; sympathetic nervous system.

Fig. 1.

Schematic of laboratory protocol. OGTT: oral glucose tolerance test.

Fig. 2.

HOMA-IR for room light (n = 10) and dim light (n = 10) conditions on Day 1 and Day 2. HOMA-IR was significantly higher on Day 2 compared to Day 1 in the room light condition versus the dim light condition. P values refer to the change from Day 1 to Day 2 between conditions (unpaired Student’s t test). The error bars represent SD.

Fig. 3.

Glucose and insulin measures from 2-h OGTT for room light (n = 10) and dim light conditions (n = 10) on Day 1 and Day 2. Within group changes in glucose levels (A and B) on Days 1 and 2 were similar between room light and dim light conditions while within group changes in insulin levels (C and D) were significantly higher on Day 2 after sleeping in room lighting in the room light condition compared to the dim light condition (General Linear Model: condition × day P = 0.034). Higher insulin levels on Day 2 in the room light condition were most pronounced at 20 and 30 min post ingestion of the glucose bolus. *P < 0.05. The error bars represent SD.

Fig. 4.

Early phase insulin response (30-min AUC) during the OGTT for room light (n = 10) and dim light (n = 10) conditions on Day 1 and Day 2. Insulin 30-min AUC and HOMA-IR were significantly higher on Day 2 compared to Day 1 in the room light condition versus the dim light condition. P values refer to the change from Day 1 to Day 2 between conditions (unpaired Student’s t test). The error bars represent SD.

Fig. 5.

Matsuda insulin sensitivity index for room light (n = 10) and dim light (n = 10) conditions on Day 1 and Day 2. Matsuda index (y-axis) was calculated as 10,000/square root of [fasting glucose × fasting insulin] × [mean glucose × mean insulin during OGTT]. Matsuda index was significantly lower on Day 2 compared to Day 1 in the room light condition versus the dim light condition. P values refer to the change from Day 1 to Day 2 between conditions (unpaired Student’s t test). The error bars represent SD.

Fig. 6.

(Top, A and B): HR during the sleep period for room light (n = 10) and dim light (n = 10) conditions on Night 1 and Night 2. (Lower, C and D): LF/HF derived from HRV analysis during the sleep period for room light (n = 10) and dim light (n = 10) conditions on Night 1 and Night 2. Beat-to-beat HR during the sleep period was averaged every 10 min across the sleep period, starting from the time at lights off. Change in HR from Night 1 to Night 2 was significantly larger in participants randomized to the room light condition (A) compared to those randomized to the dim light condition (B) (General Linear Model: condition × night P < 0.001). LF/HF was calculated on time windows with a stable signal of at least 5 min for the entire duration of the sleep period, starting from lights off. Change in LF/HF from Night 1 to Night 2 was significantly larger in participants randomized to the room light condition (C) compared to those randomized to the dim light condition (D) (General Linear Model: condition × night P < 0.019). The error bars represent SD.

Fig. 7.

Pairwise correlations between the change in LF/HF obtained from HRV and averaged across the sleep period (Night 1 to Night 2) and the change in the AUC of insulin at 30 min from OGTT (Day 1 to Day 2). A significant positive correlation between LF/HF change and the change in the AUC of insulin at 30 min was observed in participants randomized to the room light condition (A, n = 10), and not in those randomized to the dim light condition (B, n = 10). The dotted lines represent 95% CIs.