Fish don’t have eyelids so do they sleep? And if they do, how do they go about it? Yes, fish do sleep. However, some fish will do so during the day. Unlike humans however, during the sleep cycle fish may not display the characteristic brainwave patterns (like REM sleep). Some fish will rest in coral crevices for safety and camouflage or on the bottom of the ocean floor. Parrot fish have a peculiar defence system where they secrete a thick mucus cocoon almost like a sleeping bag around themselves before they sleep. The mucus potentially protects them by keeping their scent hidden from predators. Some deep sea fish never stop moving, several sharks fall into this category too. But it is unclear if fish dream, or have any of the human-like sleep cycle brain waves.
Recently several groups of scientists started using zebra fish as models to study sleep disorders. Zebra fish larvae are transparent allowing scientists to view their neuronal network while the fish are alive. Scientists found that the fish drooped their tails fins while they were motionless and spent most of the night at the bottom of the fish tank. To further confirm whether these droopy-tailed zebra fish were in fact asleep, a postdoctoral research scientist, Dr. Tohei Yokogawa, investigated to see if these fish experienced sleep rebound or the need to catch up on sleep if they were sleep deprived. Dr. Yokogawa kept the fish in a prolonged active state and found that the zebra fish that were sleep-deprived remained in a low activity sleep-like condition for longer periods of time, as though trying to compensate for lost rest. This data also confirms that fish have sleep patterns and habits that are more similar to us than we thought. Although there are still many unanswered questions, we know now that sleep comes in different forms, even while moving and having the eyes wide open!
The writer is a Ph.D. in Molecular Genetics and Genomics and is working on skin cancer at Novartis