Japanese researchers have found that even the slightest slither of light when trying to sleep could be linked to a heightened risk of depression, according to a new study published in The American Journal of Epidemiology. IFLScience reports: The reason behind this link is unclear, but the researchers believe it might be to do with the human circadian rhythm, the 24-hour cycle that tells us when to sleep and wake up, among other things, that is "programmed" by environmental factors. In the case of humans and many other creatures, light influences how much of the sleep-inducing hormone melatonin is pumped into our bodies, meaning we feel awake when the Sun rises and get sleepy when the Sun sets. This system works like a charm when there's only sunlight, moonlight, and a campfire to think about. However, the modern world is beaming with almost constant exposure to artificial light. Light at night (LAN) in a bedroom -- even a flash of a digital clock or streetlight creeping in from a crack in the curtains -- could screw with our natural sleep/wake cycle. The team behind the recent study assessed the sleep of almost 900 elderly people with no signs of depression. They found that people who slept in a room with 5 lux of light or more at night showed a "significantly higher depression risk" than those who slept in a completely dark room. For perspective, a household room with its lights on is around 80 lux and 10 lux is a single candle from 0.3 meters (1 foot) away.