Do fish with no eyelids sleep? Given that they spend their entire lives submerged in water, this is a reasonable question to ask.

The aquatic world holds many secrets, and the sleep habits of fish are no exception.

Do fish sleep?

The short answer is yes, fish do sleep. But the way they sleep differs from land-dwelling creatures. Unlike humans who have eyelids, fish don't close their eyes when they sleep. Instead, many species have a more subtle form of rest, where they appear to be less active, often hovering in one spot or resting on the bottom of the sea. Some species, like sharks and tuna, however, can roll their eyes back inside their heads when they sleep.

The sleep patterns of fish

Just like us, fish have regular sleep patterns. Some species are diurnal, meaning they are most active during the day and sleep at night, while others are nocturnal, preferring to sleep during the day and hunt at night.

For example, the colourful parrotfish, clownfish guppies and zebrafish are diurnal sleepers. They find a safe spot, reduce their activity, and rest during the night. On the other hand, the Catfish and the Moray Eel are known to be nocturnal sleepers. They find shelter during the day and become more active during the cover of darkness.

How do fish sleep underwater?

Fish don't have the luxury of a comfy bed or cosy blankets, but they've evolved ways to catch some underwater Z's. Some species, like the Parrotfish, secrete a mucous cocoon around themselves at night. This protective coating helps reduce the chances of becoming a late-night snack for predators.

Other fish may find a hidden nook in rocks or vegetation to tuck themselves away. By reducing their activity and finding a secure location, they can rest without the fear of becoming a meal while they sleep.

The decision to be a night owl or an early bird in the fish world depends on various factors like predator-prey dynamics, competition for resources, and environmental conditions. Some fish have evolved to be more active during the night to avoid predators, while others find their meals more readily available during daylight hours.

Some interesting facts about specific fish sleepers

• Dolphins sleep with one-half of their brain at a time, so they can still stay alert to danger.
• Whales and sharks sleep while drifting slowly through the water. They literally "sleep on the move."
• Some fish, like the remora, attach themselves to bigger animals while they sleep.