Urban lights may prevent fish from “closing their eyes” at night, study says

Although they don’t have their eyelids closed, fish do “sleep” — just like humans, and sleep is vital to their health,media New Atlas reported. But according to a recent study, city lights may prevent them from falling asleep at night. The study was led by Professor Werner Kloas of the Institute of Freshwater Ecology and Inland Fisheries in Leibniz, Germany.

Urban lights may prevent fish from "closing their eyes" at night, study says

His team began his research with four captive groups of European mackerel. For 10 days, each night, one control group of fish was reared in complete darkness, while the other three groups were exposed to 0.01, 0.1 and 1 lux.

For reference, on a moonless sunny night, the natural light intensity is less than 0.001 lux, while the full moon can reach 0.3 lux on a night. Clouds over the city reflect the city’s glow by up to 1 lux or higher, while direct lighting on the streets can reach 150 lux.

After 10 days, all herring is checked for melatonin levels every 3 hours within 24 hours. Melatonin is a hormone produced by vertebrates such as fish and humans based on perceived ambient brightness. In general, the lower the light level, the greater the amount of melatonin produced — the body sets its own circadian rhythm based on these levels, including the start time of sleep.

In addition, scientists have found that at night fish are exposed to brighter light, the less melatonin they produce. The pace of melatonin production remains the same throughout the day, although previous studies have shown that it is also affected by stronger light intensity — say, 10 to 100 lux — because the amount of melatonin produced during the day and night is essentially unchanged.

While it’s not clear how long different groups of herring slept during the test, Kloas says the decrease in melatonin production could in itself lead to problems with functions such as immune defense, growth and reproduction.

A paper on the study was recently published in the journal Environmental Pollution.