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A single night of light exposure during sleep impairs glucose metabolism via activation of the “fight-or-flight” response.

Light is the primary signal that entrains the body’s master clock to set its 24-hour circadian cycle. Consequently, the body is synchronized to external light-dark cycles. In recent decades, exposure to light from artificial sources has increased, especially during the evening and nighttime hours, with negative effects on human health. Findings from a recent study suggest that a single night of light exposure impairs glucose metabolism via activation of the sympathetic nervous system.

The sympathetic nervous system is a division of the autonomic nervous system. It responds to both endogenous and exogenous stressors and is widely referred to as the coordinator of the body’s “fight-or-flight” response. The outcome of sympathetic nervous system activation is an increase in heart rate, cardiac output, and blood glucose levels, as well as other physiological responses that prepare the body for action. Evidence suggests that increased sympathetic nervous system activity alters sympathovagal balance (the balance between the sympathetic and parasympathetic nervous systems), driving poor heart rate variability.

The investigators recruited 20 healthy adults (average age, 26 years) who did not have sleep disorders. Participants spent two nights in a sleep laboratory, where they ate all their meals and went to bed at their habitual times. Half of the participants spent one night in dim light conditions (less than 3 lux, very dark) and one night in room light conditions (100 lux, from four 60-watt incandescent bulbs). The other half spent both nights in the dim light conditions. Participants provided blood samples and underwent oral glucose tolerance tests each morning.

The investigators found that participants who were exposed to room light conditions during sleep had increased nighttime heart rate, decreased heart rate variability, and increased morning insulin resistance, compared to when they slept in a dark room. They also spent less time in deep, slow-wave sleep.

These findings suggest that a single night of exposure to room light during sleep impairs glucose metabolism via activation of the sympathetic nervous system. Learn how light from devices impairs sleep in this clip featuring sleep expert Dr. Matthew Walker.

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