Under the Microscope: Even though we spend about a third of our lives asleep, science does not yet know sleep's exact functions, writes Prof William Reville.
Researchers are making progress, however, as described by Jerome Segal in Scientific American.
For many years it was thought that our relative immobility and inattentiveness to our surroundings during sleep simply reflected a temporary shutdown of most brain activity. This view changed in 1953, when it was discovered that sleep is also characterised by periods of rapid-eye-movement, or REM, sleep.
REM sleep, which occurs in all land mammals, alternates in a regular cycle with non-REM sleep, also called quiet sleep. The neurons, or nerve cells, in the brain generate and transmit electrical signals from one to the other. When we are awake most of the neurons work at near maximum rates. During non-REM sleep most neurons in the brain stem, just above the spinal cord, greatly reduce their activities. Neurons in the cerebral cortex reduce activity only slightly, but their pattern of activity changes. In the awake state each neuron behaves independently. During non-REM sleep adjacent neurons fire synchronously and consume less energy. Heart rate and breathing are very regular and little dreaming is done in non-REM sleep.
During REM sleep the brain behaves in many ways like the waking brain. Most of the brain cells are quite active, and the overall consumption of energy is as high as while awake. Vivid dreams occur during REM sleep, and the brain's motor systems are frequently activated. These motor systems, apart from those that control the eye muscles, are inhibited from causing movement during REM sleep. The brain stops releasing neurotransmitters that would activate motor neurons to contract muscles. Breathing and heart rate become irregular during REM sleep, and body temperature is more coarsely regulated. Males often get erections, although most dreaming does not involve sexual imagery.
When investigating the function of a biological behaviour, it is often instructive to observe the effect of completely inhibiting the behaviour. Rats totally deprived of sleep lose weight despite greatly increased food consumption and die within 10 to 20 days. They live longer if allowed to sleep normally but totally deprived of food. There is a rare human brain disease called fatal familial insomnia that results in death after several months.
Sleeping is clearly essential, but how does it do its job? A comparison of sleeping patterns across different types of animals yields an important clue. Size is the major determinant of the amount of sleep an animal needs. Bigger animals need less sleep. An elephant, for example, sleeps for three hours a day, a human for eight hours, a dog for 10 hours, a cat for 12.5 hours, a ferret for 14.4 hours and an opossum for 18 hours.
The reason for the different sleep requirements apparently is that smaller animals have faster metabolisms and higher body and brain temperatures than larger animals. Metabolism generates chemicals called free radicals that are damaging to cells. Segal proposes that the main function of non-REM sleep, with its lowered metabolic rate and brain temperature, is to repair damage done during wakefulness. Segal's laboratory has produced evidence that sleep deprivation damages rats' brain cells.
This repair hypothesis does not explain REM sleep, because in this phase the brain cells are as active as during waking. Segal proposes a mechanism to explain the function of REM sleep based on neurotransmitters called monoamines, which are not released in this sleep phase. When a neurotransmitter moves from one neuron to another it docks with a receptor on the second neuron. Continuous release of monoamines desensitises the receptors. The cessation of monoamine release during REM sleep may allow the receptor system to rest and recover sensitivity. A high level of sensitivity during waking hours is crucial for mood regulation, which depends on the smooth synchronisation of transmission and reception.
Animals deprived of REM sleep compensate when given the opportunity by undergoing more than the usual amount of REM sleep. It used to be thought that depriving subjects of REM sleep eventually produced insanity, but this has been disproved. Interestingly, REM-sleep deprivation can alleviate clinical depression. This may be because the normal REM decline in monoamine concentration does not occur. The most popular theory of depression is that it is caused by underactive monoamines.
Some researchers feel REM sleep may consolidate memory, but the evidence is weak. People with brain damage that prevents REM sleep can have normal memory. And species' learning ability is unrelated to the duration of REM sleep. Dolphins have little or no REM sleep but have substantial reasoning and learning ability.
The main determinant of how much REM sleep an adult of any species requires seems to be how immature the offspring is at birth. The platypus is born defenceless, blind and dependent on its mother. The adult platypus spends about eight hours a day in REM sleep. A dolphin must be able to follow its mother and avoid predators from birth. Adult dolphins show almost no REM sleeping. Humans adults spend one and a half to two hours in REM sleep each night
William Reville is associate professor of biochemistry and director of microscopy at University College Cork.