Do all animals sleep?
Do all animals sleep? And do any plants or bacteria?
Chris Smith put this to Royal Papworth Hospital's Nick Oscroft...
Nick - All mammals and birds sleep as far as we’re aware. Reptiles also sleep but it’s a bit more tricky to measure, but we think they do. As you get to more primitive organisms, we know zebrafish have a resting state. There’s lots of scientific data about that and they’re often used in experiments to investigate sleep. Plants do have a light/dark cycle, but they don’t have a central nervous system so I’m not sure if they specifically sleep, but they clearly are sensitive to their environment. Then we get down to very primitive things like bacteria, most things exhibit a resting phase. Whether we could call that sleep in bacteria I’m not sure. But what I would call sleep in terms of it being quite a regulated process that happens at a certain time that you have to catch up with if you miss it, probably is something that happens more in the higher organisms rather than all the way down the food chain as it were.
Chris - What about the sorts of creatures like marine mammals that apparently can sleep half their brain at a time? Is this true or is it a myth and, if it is true, how do they do it?
Nick - There is evidence that they do have so called ‘hemispheric sleep’ in marine mammals as you state. But, also, other animals that need to keep swimming, for example, sharks because the need to keep seawater going through their gills to keep them oxygenated. You can make cats have hemispheric sleep if you do the right operation on them as well in scientific experiments. So yeah, that is true.
Chris - It’s called hemispheric sleep because just one hemisphere of the brain is deactivated into a sleep state at one time?
Nick - Yes, and that has been studied. Generally they tend to have non-REM sleep rather than REM sleep, and the reason for that being that when you go into REM sleep your body tends to actively paralyse you. Half of you becoming paralysed whilst you’re swimming probably wouldn’t help.
Chris - But bacteria: loose definition of sleep but they do have a metabolic bit of downtime?
Nick - Yes.