Sleeping is such an essentially absurd activity. We run around frantically during the day, then suddenly we need to lie down and keep quiet for a few hours, only to get up and run around once more. It's a curious cycle that needs to be repeated at regular intervals throughout a healthy life. There is a misconception that sleep is a restful state - the opposite of being awake - simply because we don't move much during sleep. The truth is that being asleep is not akin to being unconscious at all; it is very much an altered state of consciousness. In fact, we spend our lives in three states of consciousness: wakefulness, NREM sleep and REM sleep. Although two of these states are lumped together as 'sleep', they are very different from one another. NREM (non-rapid eye movement) sleep is divided into four progressively deeper stages, culminating in slow-wave sleep which is characterized by rolling delta brain waves on an EEG. Ho-hum. So far sleep doesn't seem that exciting. But as soon as the brain enters REM sleep, dynamic changes can be observed. To the researcher monitoring a sleeping person's EEG, the brain waves would suggest that the sleeper has woken up after the 90 or so minutes spent in NREM sleep. Heartrate and breathing are irregular. The body is responding to some sort of stimuli. Yet the sleeping individual is definitely not awake and lies still, the only movement being that of the eyes under closed lids. The stimuli are not entering from the outside environment through the senses, but are internally generated. It is the only instance when the brain hallucinates and we consider it normal and healthy. This psychosis is more commonly referred to as dreaming. REM sleep is a fascinating but difficult aspect of life to study. What is it for? Why is the brain so active during REM sleep? Why do we dream? One thing is clear: sleep is essential for survival. Sleep heals wounds and stimulates the immune system. We know that reduced sleep significantly increases one's risk of getting involved in accidents, simply due to fatigue and impaired responsiveness. Total, extended deprivation itself can also be fatal: in one study, lab rats died after 11 to 32 days of being denied any chance of sleep, and from 16 to 54 days if selectively deprived of REM sleep only.
The psychedelic splendour of REM sleep! That warm and inviting abyss of dreams. Turn on, tune in, drop out; no acid required. REM atonia ensures that we don't act out our dreams. This is fortunate for those sharing a matress with a fellow dreamer. The motor neurons that control muscle movements are selectively inhibited as REM sleep is initiated. Sometimes this inhibition is triggered before REM is initiated, or persists for some time after a REM phase. When this happens, you find yourself at the interface between sleep and wakefulness, unable to move. Welcome to the land of hypnagogia, where input from the senses is taken by the brain and stretched, distorted, and expanded through iterations of its own design. This is actually the state you normally pass through on the way to dreamland. However, when REM atonia is accidentally coupled with these sensations, people often feel a 'presence' in the room with them, or a fear-inducing pressure on their chest, an inability to breathe. Voluntary breathing is suppressed under REM atonia and is replaced by involuntary breathing controlled by the brainstem, see. Nothing to fear. Yet we don't want to relinquish that conscious control. The anxiety brought about by such incidents of sleep paralysis has given rise to a myriad of legends about crushing demons, maras and the incubus. And all reports of alien abduction from bedrooms, I expect. Apart from the unlucky few who experience it every night, the majority of sleepers will experience sleep paralysis at most once or twice in their lives. So don't be alarmed and enjoy the ride - after all, you engineered it yourself.
REM sleep disorders are teaching scientists all sorts of things. From how the consolidation of memories works by reinforcing them with biochemical loops in new connections between neurons (you may have to read that again), to fascinating breakthroughs in the relationship between sleeping and eating. Yes, the two are connected. If our requirements for energy, shelter and procreation were somehow met by proxy, would we spend our whole lives asleep? Perhaps that's our true baseline state of being. Apart from sophisticated genetic mechanisms that ensure we live our lives according to circadian rhythms, there are others that trigger wakefulness in response to changes in energy reserves. Small proteins called orexins (or hypocretins, depending on which research team you talk to) are produced by a couple of cells in the hypothalamus for just this purpose. These proteins are the brain's own breakfast bell. Interestingly, when this bell malfunctions, either through mutations in the genes for the orexins themselves or in the genes for their receptors, narcolepsy results. This intriguing sleep disorder really warrants a post of its own. I plan to share my thoughts on this remarkable story in a future piece. It's a tale of obese mice, sleepy dobermans, collapsing teenagers and some of the most elegant experiments in all of molecular biology.
All mammals sleep; we all require it to live. So I'll leave you with some final thoughts about whales. Cetaceans (that's whales and dolphins to you non-biology types) never enter NREM sleep with both hemispheres of their brains at the same time. This is called unihemispheric slow-wave sleep. In fact, they sleep with only one eye closed - the left when the right hemisphere is asleep, and vice versa. Good thinking, to prevent sinking. And drowning, that certainly wouldn't be conducive to the survival of the species at all. Bottlenose dolphins sometimes rest on the bottom, or seem to just float for a bit. More commonly, they slowly swim counterclockwise while sleeping, even when switching brain hemispheres. Smaller dolphins and porpoises are always on the move from birth until death; they are never immobile, the hallmark of terrestrial sleep. Sleep is anything but a restful, unconscious state. Here is the weird thing, though: nobody has ever published accounts of REM sleep in cetaceans; they are the only mammals studied in which this state has never been observed. How do they survive without the third state of consciousness? Why does REM sleep exist, then? How does learning and the consolidation of memory occur? Do dolphins dream?