Recently, I had an enjoyable chat with Graeme Hill on the subject of sleep. Also on the show was Karyn O'Keeffe, whose research interests are with the physiology of sleep (& the lack of it). My segment focused on the evolution of sleep and yes, I did quite a bit of reading in preparation!
Sleep is mediated by a chemical messenger, melatonin. The onset of darkness triggers the pineal gland to release melatonin, which alters the activity of the neurons in our brains, resulting in sleepiness. Exposure to light destroys melatonin, & so we wake. (Hence the concern about the use of tablets, smartphones etc late into the evening, because of the potential for this to upset the normal sleep-wake cycle.)
One of the things Graeme & I discussed was the evolution of sleep – when did this particular aspect of physiology & behaviour evolve? One of my favourite science writers, Carl Zimmer, wrote about this in 2014. He linked to a study suggesting that sleep may have evolved around 700 million years ago, on the basis of research into gene expression in a marine worm called Platynereis dumerilii.The larvae of these worms move up & down in the water column on a 24-hour cycle (a circadian rhythm), and while the 2-day-old larvae used in the study certainly don't yet have eyes, they do produce and respond to melatonin. Cells on the upper suface of these tiny larvae detect changes in light intensity, and with the onset of darkness, they turn on melatonin production. This stops the beating hairs (cilia) that allow the animals to swim towards the suface, and so they sink slowly down towards the depths. But not so deeply that the light-sensitive cells can't detect dawn's light, which destroys the melatonin and so the larvae swim upwards once again. (Apparently it's even possible to give them jetlag!)
"Well", said Graeme, "why do we sleep? It seems on the face of it a very risky business given the prevalence of nocturnal predators."
And it's a good question. Why would sleep be selected for, given that you'd think it would make animals more vulnerable to predation? It must have some strong adaptive advantages, to outweigh that risk! Some of the possibilities are canvassed in this short piece in Scientific American, which notes that patterns of sleep and wakefulness vary enormously between species, and also within species depending on time of year – think of hibernation, for example. (Even that varies between species in terms of sleep duration: hummingbirds go into a form of torpor that may last less than a day each time.). The author, Christopher French, suggests that "sleep and related states provide periods of adaptive inactivity" – that is, that a lack of activity can also provide a selective advantage:
Most likely sleep evolved to ensure that species are not active when they are most vulnerable to predation and when their food supply is scarce.
He gives the example of a bat that sleeps around 20 hours a day, rousing to hunt insects that are active in the dusk. Searching for these insects during the day would be fruitless, but would also expose the bats to diurnal predators. But sleep must also be important in some way for the brain, as it's so markedly affected by a prolonged lack of sleep. (And now, I want to know how the melatonin thing works in species like this, that are active at night and sleep during the day. Apparently even nocturnal animals produce the most melatonin at night.)
A few years back, researchers asked the question, "Is sleep essential?" (Cirelli & Tononi, 2008). (There's an excellent lay summary here – teachers would find it very useful.) I found the paper interesting because, in preparing to test their null hypothesis (that sleep is not essential), the authors first defined sleep:
Sleep is a reversible condition of reduced responsiveness usually associated with immobility.
Most work on sleep has been done in mammals & birds, but it seems that even the humble lab workfly, Drosophila, can be said to sleep: these little flies have periods when they become less responsive to stimuli; if they're forced to stay active, 'sleep pressure' increases; patterns of sleep & wakefulness change with age; hypnotic and stimulant drugs affect their activity; and gene expression in the brain changes with periods of sleep and wakefulness.
Cirelli & Tononi's paper is open access & well worth reading. They feel that the available evidence doesn't support claims that bullfrogs, for example, do not sleep. Dolphins and other marine mammals, which are constantly moving, still appear to enter a form of sleep – in one hemisphere of the brain at a time – a statement supported by evidence of changes in brain wave activity.
They also discuss the effects of sleep loss – in rats, flies, cockroaches, and humans, prolonged sleep deprivation can ultimately be fatal. (This was the point at which Graeme told me about something called fatal familial insomnia, which sounds awful and is apparently one of the group of prion diseases, along with things like scrapie.) But well before that point, sleep deprivation affects performance, particularly in terms of cognitive performance. So, students take note! an all-nighter or two ahead of major exams is unlikely to work in your favour.
Cirelli C, Tononi G (2008) Is Sleep Essential? PLoS Biol 6(8): e216. https://doi.org/10.1371/journal.pbio.0060216