Do Fish Sleep?


I have got a question for you Helen here. It is from Glenn Colson who says: 'Do fish sleep?'

Helen Scales - Well I think Diana talked about on Question of the Week a couple of weeks ago about whether fish have eyelids and they don't. So can they sleep? The answer is yes, they can but it also depends on how you define what sleep is because in humans the transition into sleep involves changes in the patterns of brainwaves in an area of our brain called the neocortex and fish don't really have the same development in their neocortical region as mammals, really.

So it's kind of difficult to say in the same way what sleep really means for fish but they do. If you count up things like a reduced metabolic rate and slowing down activity then they do seem to sleep and Zebra fish, which are fresh water fish, have been studied in the laboratory. If you watch them, and I think this is rather sweet idea that. You can watch them during the day and they will actually sort of doze and their tails droop down and they stop moving around at certain times of the day and they do seem to sleep.

That's one thing and in Antarctica, actually only last year they discovered the first hibernating fish and apart from mudfish that aestivate and keep themselves alive when things dry out. In Antarctica they actually slow down and stop feeding and their heart rate slows down. They used heart rate monitors on these fish to see what was going on and we think that it was probably because it gets darker in the winter in Antarctica. In fact it gets completely dark because there's no sun and they find it hard tracking down their prey because this prey is still out there but they are visual hunters and without the light around they find it very difficult to find their food so they sort of stop.

Chris Smith - They just sleep.

Helen Scales - And they're a bit like bears. Some bears don't sleep the entire winter. They will actually kind of wake up every few weeks, have a little wander around, find a bit food, go back and sleep again. That's what these Antarctic cod seem to be doing as well.

Chris Smith - It's quite interesting because Glenn has written a few things he spotted and this question was provoked by things he actually saw himself. He says: 'A few years ago I was on a night dive in the sea of Cortez and we actually think we saw a fish sleeping in a number of different ways. There were fish in caves or nooks that were awake but only slightly responsive. There were also fish lying on the sea floor looking like they were dozing soundly and they weren't at all perturbed by our diving light and there were others that had wrapped themselves in a cocoon of slime that seemed to act almost like a protective blanket.'

He says, 'I suspect they have nightmares as well because as we watched a large moray eel swam slowly across the bottom of the seafloor stopping to sniff prospective meals as they were snoozing and then choosing his favourites but the moray left the blanketed fish alone.' A wrass he believes (which he found interesting) because he said that the eels and they normally find it quite tasty.

Helen Scales - Yes, they are parrot fish and they produce this kind of sleeping bags and we do think that stops them being able to smell, the predators can't smell them but there are other fish that come awake at night. In fact if you go night diving you see a completely different eco system really because the ones that were sleep during the day tend to come out. They are often coloured red because they actually, that means they can't be seen at night because the red colour is absorbed when light is absorbed in the water. So you see kind of a shift change in day and night fish when you go night diving.

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