For something we spend so much time doing, we still don鈥檛 really know what sleep is for. It鈥檚 clear that we need it. Our ability to perform tasks and make decisions is greatly impaired by a lack of sleep, as anyone who has had a restless night will attest. But it鈥檚 not just humans who are so reliant upon shut-eye. Animals need sleep too, even birds that fly continuously for months. So what鈥檚 sleep for?聽 And why can鈥檛 we stay awake indefinitely? In this episode of Science with Sam, the first in the new series, we explain the science of sleep.
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Video transcript
We spend about a third of our lives asleep. And if we miss out on it, we get physically sick. Clearly sleep is extremely important, but why? What鈥檚 going on in our brains while we鈥檙e out of it? Why can鈥檛 we stay awake all the time? And how can we make sure we鈥檙e getting enough?
It鈥檚 not just us humans who need our shuteye. Bees need about as much sleep as we do. If you keep them up at night, they are badly affected. This is a normal bee doing the waggle dance, a sort of insect version of charades that tells other bees where to find food. And here鈥檚 a sleep-deprived bee. Its moves are less precise; there is more variation in the angle of its dance, and as a result, it鈥檚 giving bad directions. If you miss a night鈥檚 sleep and then try to dance, or play charades, you鈥檒l be like this too.
Even animals that spend weeks or months flying continuously can鈥檛 do without sleep. Researchers have put wearable brainwave recorders on great frigatebirds to confirm the common belief that birds can sleep while flying. They napped in short bursts of about 12 seconds during long flights, getting a total of 41 minutes per day, usually sleeping one half of the brain at a time. When they were back on land, they would sleep for more than 12 hours a day.
So birds do it, bees do it, even enervated fleas do it. Sleep is thought to be common to all animals with a central nervous system, but it turns out to be even more ubiquitous than that.
Even jellyfish, which have a simple, diffuse nervous system and no centralised brain have been found to enter a sleep-like state at night, and become dozy the next day if their rest is interrupted.
But although sleep seems to be common to all animals, we still don鈥檛 really know what it鈥檚 for.
Many attempts to explain sleep suggest it鈥檚 important for repair or maintenance of the brain. It was once thought that some kind of toxin builds up in the brain during our waking hours which makes sleep irresistible when it gets to a certain level. Such a substance has never been found.
A modern version of this idea says that during the day we deplete supplies of large molecules essential for the operation of the brain, like proteins, RNA and cholesterol, and that these are replenished during sleep. It has been found in animals that production of these molecules increases during slow-wave sleep, but it hasn鈥檛 been shown they control sleep.
Another school of thought is that sleep isn鈥檛 just a maintenance thing 鈥� it must have some unique, active function. Perhaps the best-known idea is that sleep helps us consolidate memories, moving the important details into long-term storage. Many experiments have shown that our memories are stronger if sleep takes place between learning and recall.
One popular theory relates to both neuronal maintenance and memory processing. It concerns the synapses, connections between neurons. When we form new memories, they get stronger. But if we keep on making new memories while we鈥檙e awake, strengthening those synapses is very expensive in terms of energy. So during sleep, the slow cycles of brain activity lower synaptic strength across the board, maintaining memories but saving energy.
While we鈥檙e snoozing, we cycle through different stages of sleep and it鈥檚 likely that each stage has different functions. The first few cycles in a night are important for deep sleep, which seems to be most important for brain maintenance. Later cycles include more REM sleep, which stands for rapid eye movement. This is the stage when we have vivid dreams, and animal studies suggest it鈥檚 important for learning. On the other hand, some antidepressants suppress REM sleep, and they don鈥檛 seem to affect people鈥檚 memories.
Whatever the reason for sleep, it鈥檚 undoubtedly very important for our wellbeing. Not getting enough affects your mood, your decision-making, your immune system and your metabolism. Inadequate sleep is a risk factor for obesity, type 2 diabetes, and a host of mental and neurological conditions such as Alzheimer鈥檚, schizophrenia, depression and bipolar disorder.
It鈥檚 widely claimed that we need 8 hours a night, but that isn鈥檛 set in stone. Tribal cultures with no access to electricity sleep for 6 or 7 hours a night. Even in developed societies, it seems to be the case that 7 is enough for most people.
Some people claim to get by on much less than everyone else. These people probably are sleep-deprived, but have just got used to it, although some genetic variations do seem to help with recovery from sleep deprivation. Napping can help your alertness, but a full night鈥檚 sleep is much better, as it lets you go through several sleep cycles.
If you worry you鈥檙e not getting enough, here are a few tips that may help.
- Go to bed and wake up at the same time every day.
- Make sure your room is cool, around 18.5 degrees. Your body temperature drops at night, but if it鈥檚 too cold that can be bad for sleep too.
- Dim your lights before bed. Light suppresses the secretion of melatonin, a hormone that makes us feel sleepy. Blue light, which is produced a lot by screens, is particularly bad, so avoid phones and laptops before bed. TV is not so bad if you鈥檙e sitting a good distance away.
- Stay off the booze. Alcohol is a sedative, and sedation is not sleep. Even a drink in the early evening can affect your sleep in the second half of the night. Likewise, caffeine should be avoided after 1pm and smoking at any time of day is linked to less sleep.
- Try drinking sour cherry juice instead. It鈥檚 high in melatonin, and a study found that people who drank it twice a day got 34 minutes more sleep at night.
We all know what it鈥檚 like to be the badly-dancing bee, robbed of our slumber, unable to function like we鈥檙e supposed to. On top of that, we鈥檙e discovering that poor sleep can predispose us to serious health problems in the long run, including Alzheimer鈥檚 disease.
That鈥檚 a scary thought, but it gives us another incentive, if we needed one, to prioritise sleep. It might just help your brain stay healthy a little longer. Sweet dreams.
