Why do organisms sleep?
“Methought I heard a voice cry, “Sleep no more!
Macbeth does murder sleep!” the innocent sleep,
Sleep that knits up the ravelled sleave of care,
The death of each day’s life, sore labor’s bath,
Balm of hurt minds, great nature’s second course,
Chief nourisher in life’s feast.”
Macbeth spoke these words to his lady with strained desperation as he brooded over his murderous deed. Sleep is a curious contradiction – slumber is both vulnerable and comforting. However, assuming you’re not wrapped up in any malicious political assassination plots, it’s more often the latter.
The “how” of sleep has been widely researched – from the progression of the stages of “slow-wave” sleep and the details of deep slumber (REM), to the heavy physical and cognitive impairments resulting from sleep deprivation.
The “why” of sleep is more of a mystery. There are countless theories about the purpose of sleep. It has been implicated in the secretion of vital hormones, neurogenesis (the creation of new neurons), memory consolidation, and immune system optimization. However, in a recent article in Nature’s special review issue celebrating Darwin’s 200th birthday, Jerome Siegel of UCLA argues that these effects of sleep don’t explain the considerable variation in sleep patterns across the animal kingdom.
Mammals range from less than 3 hours of REM sleep a day to more than 8 hours. Some hibernate (brown bears, ground squirrels), while others seldom doze (giraffes, walruses). Dolphins enter an intriguing REM imitation called “ultra slow-wave sleep,” where they alternate between brief bouts of lazy floating and slow swimming (though they remain very responsive to environmental stimuli). When fur seals slumber deeply they turn off one hemisphere of their brain at a time, with one flipper keeping balance while the other remains motionless.
Non-mammals show similar levels of variation. White-crowned sparrows sleep very little, and sometimes not at all when they’re migrating. Hummingbirds enter a hibernation-like state called “torpor,” which is marked by a relatively short period of extremely low metabolism (think of it as a shorter but more “turned-off” hibernation). Reptiles have not been definitively shown to enter REM sleep, and their sleeping habits are often dependent on temperature. Even non-animals exhibit sleep-like behaviors – many deciduous trees are dormant in certain seasons, parasites often enter suspended cystoid phases during their life cycle, and one species of yeast was brought back from a 45 million year slumber and brewed into a beer.
Siegel’s argument is that the variation mentioned above (which is a small sample of the data he cites) makes it unlikely that sleep has some widely-applicable neurological function, which is a common belief. Bears have unexceptional immune systems, but go into wildly long periods of deep hibernation. Lizards can learn and have memories, but show no REM sleep at all. Dolphins don’t have growth hormone deficiencies, but are moving around their entire life. Trees certainly don’t have neurogenesis (or any neurons at all) but enter states of seasonal sleep-like dormancy.
There are certain obvious functions of sleep that work across species and explain variation. When sleeping, animals remain in one spot, thus decreasing the chance of running into a hungry predator and this defense mechanism is especially practical when the animal sleeps in a protected cave or nest (perhaps King Duncan should have taken a cue from these animals). Furthermore, one aspect of sleep is absolutely universal across all living things – it saves energy. Siegel likens sleep to “turning off the lights” when you leave a room – animals only want to spend the energy they need to spend to carry out vital behaviors (eating, mating, rearing children, etc). The more energy the animal burns, the more it needs to be out and about foraging for food, at risk for both predation and starvation. This idea is supported by the variation of sleep patterns in the animal kingdom, and these patterns are dependent on environmental factors – brown bats sleep 20 hours a day to avoid bird predation and they forage vigorously for the other 4, Giraffes sleep very little as they need to keep moving to avoid open-plain predation and eat plenty of low-nutrition flora, and lions sleep most of the day to conserve vigor for the demanding hunt.
Sleep is an energy saving mechanism, or as Siegel puts it, “adaptive inactivity.” It may be a rather simple answer to the “why we sleep” question, but it covers the bases and makes convincing adaptive sense:
“Why would some species need so much more of the mysterious restorative process that has been proposed to determine sleep duration than other species?…Sleep is best understood as a variant of dormant states seen throughout the plant and animal kingdoms and that is itself highly adaptive because it optimizes the timing and duration of behavior.”
That is not to say the other proposed functions of sleep aren’t significant, but they are less likely to be the reason sleep evolved in the first place. Simple energy conservation and risk avoidance tell a compelling story – sleep is, above all, the “Chief nourisher in life’s feast.”
I look forward to the unearthing of Dr. Shakespeare’s methodical research on the correlation of foraging patterns, food intake, and REM duration in soon-to-be assassinated medieval Scottish kings.