Brighter nights, greater health risks: intensity-dependent disruptions by nighttime light exposure on sleep and metabolism in diurnal zebra finches.

Recent studies have shown that dark nights are crucial for the maintenance of both body and brain health. Given unavoidable exposure to lighted night environment at the present time of increasing usage of light at night, it is critical to identify the nocturnal illumination that would still be less harmful. The present study addressed this, by examining daily activity-rest and nocturnal sleep patterns, and metabolic effects in diurnal zebra finches exposed to an equinox photoperiod (12:12 light: dark cycle) containing an identical light intensity during day (L = ~ 150 lux) and no light (0 lux) or three increasing dim light illuminations at 2-, 5- or 8-lux light intensities (n = 7 per condition). Illuminated nights caused disruptions in diurnal activity and feeding patterns, nocturnal sleep, and metabolism. In particular, birds under lighted nights showed an increased night activity and eating bouts, without a significant change in the 24-h activity and food intake, and fragmented sleep with frequent and smaller bouts. At the end of 3 weeks, birds were fatter and gained body mass under lighted night than those in dark night. We also found increased blood glucose levels and reduced oxalate, but not corticosterone, and concurrent effects on the expression of genes associated with glucose and lipid metabolism. Most importantly, however, the severity of deleterious effects paralleled the increasing nocturnal illumination, i.e., effects under 2 lux < 5 lux < 8 lux light intensity at night, compared to the dark night (0 lux). These results argue for the usage of reduced light intensity in situations where night light is unavoidable to minimize the health risks to animals, and perhaps humans in an urbanized environment.