Sleep Is To Save Energy

Short sleepers, about 1% to 3% of the population, function well on less than 6 hours of sleep without being tired during the day. They tend to be unusually energetic and outgoing. (more)

What fundamental cost do short sleepers pay for their extra wakeful hours? A recent Science article collects an impressive range of evidence (quoted below) to support the theory that the main function of sleep is just to save energy – sleeping brains use a lot less energy, and wakeful human brains use as much as 25% of body energy. People vary in how much sleep they are programmed to need, and if this theory is correct the main risk short sleepers face is that they’ll more easily starve to death in very lean times.

Of course once we were programmed to regularly sleep to save energy, no doubt other biological and mental processes were adapted to take some small advantages from this arrangement. And once those adaptations are in place, it might become expensive for a body to violate those expectations. One person might need sleep because their bodies expect them to sleep a lot, but another body that isn’t programmed to expect as much sleep needn’t pay much of a cost for that, aside from the higher energy cost to run the energy-expensive brain more.

This has dramatic implications for the em future I’ve been exploring. Ems could be selected from among the 1-3% of humans who need less sleep, and we needn’t expect to pay any systematic cost for this in other parameters, other than due to there being only a finite number of humans to pick from. We might even find the global brain parameters that bodies now use to tell brains when they need sleep, and change their settings to turn ems of humans who need a lot of sleep into ems who need a lot less sleep. Average em sleep hours might then plausibly become six hours a night or less.

Those promised quotes:

Sleep … appears to be nearly universal among birds and mammals, leading to the assumption that sleep serves an unknown but vital physiological function. However, no function that can explain the huge variation in sleep times within and between species has yet been firmly identified, although many candidates, including reversal of oxidative stress, memory consolidation, extension of life span, and removal of various neurotoxins, have been proposed. …

Sandpipers engage in complex courtship displays and aggressive defense of potential mates over 3-week periods. … During this time, the males show no reduction in activity or degradation of performance despite little or no sleep…. Killer whale and dolphin mothers and their calves are continuously active with eyes open for 6 or more weeks after birth. No rebound of inactive behavior follows. During this period, the neonates’ brain and body grow to their prodigious size and capacity without any apparent need for sleep-linked detoxification. Adult dolphins working for reward can accurately discriminate between visual stimuli presented at 30-s intervals on their left or right sides, 24 hours per day, for as long as 5 days. During this time, their performance shows no progressive decline. No rebound of inactivity follows the session. By contrast, humans whose sleep is interrupted on a similar schedule are dramatically impaired . … Migrating birds greatly reduce sleep time with intact learning abilities and high rates of performance, with no subsequent sleep rebound. …

If male sandpipers that are continuously active during the breeding season clearly leave more offspring than males who sleep, why hasn’t natural selection eliminated the sleepier males? An attractive explanation is that the active males are more likely to deplete caloric reserves. … In conditions of food scarcity, the birds that most greatly depleted caloric reserves during mating will be least likely to survive to the next mating season. …

Humans … of similar age, sex, and body build can have very different sleep times. They can also vary in their response to sleep loss, with some being highly impaired, unable to resist sleep, and others showing high levels of functioning despite sleep loss. The effect of sleep deprivation on performance is not strongly related to baseline sleep duration. Furthermore, human sleep duration is not linearly related to health, with both high and low values being linked to shortened life span. Death of rats due to sleep deprivation may be related to stress rather than sleep loss. Sleep deprivation has not been reported to cause death in pigeons, mice, or in rats deprived by techniques that do not involve waking them frequently at sleep onset. Fatal familial insomnia can cause death in humans, but sleep loss does not appear to be responsible. …

So why do most of us feel so poorly when we reduce our sleep time? Natural selection has imposed a certain amount of sleep on us to restrict activity to appropriate times of day and to reduce long-term nonvital energy expenditure. The pressure to sleep operates by reducing brain activity. Although individuals with naturally short sleep times are not at elevated risk compared to those with naturally long sleep times, repeated sleep deprivation below the body’s programmed level is stressful and likely to impair health. Certain hormonal processes are linked to sleep. However, these are not universal, but rather are species and age specific.

Sleep duration varies enormously across species, with total sleep amount ranging from 20 hours per day in the big brown bat to 2 hours per day in the horse. However, attempts to correlate sleep time with various parameters do not support any sleep physiology theory. But species that eat food with low caloric density (e.g., herbivores) sleep less than those eating more nutritionally dense foods (e.g., carnivores) … Animals will achieve a selective advantage in reducing brain energy consumption by sleep, but only if they have safe sleeping sites, such as under-ground burrows. Accordingly, large prey animals that do not have safe sleep sites do not sleep much and sleep very lightly.

In addition to mating and migration, sleep can also be reduced during food shortages, presumably to allow animals to invest more time in searching for the available food. Species whose environment has a severe seasonal variation in food availability have evolved to increase sleep during periods of food shortage and decrease sleep when food is available. (more)