Some animals have evolved remarkable adaptations to be able to sleep and survive in an otherwise inhospitable environment. Consider advanced mammals like dolphins and whales for instance. How do they sleep and not drown?
These animals have developed the truly amazing capability of unihemispheric sleep, in which one side of the brain sleeps while the other side is awake. Unihemispheric sleep allows dolphins and whales to sleep on one side of their brain while the other side stays alert. This enables them to continue swimming and surfacing to breathe while part of their brain sleeps.
Birds that make long transoceanic or migratory flights, such as mallards, do the same thing – sleep on one side of their brain, then switch to the other. This capability allows these animals to sleep while simultaneously tracking other group members and watching for predators.
Interestingly, some research suggests humans may have also a vestige of this capability, which might help explain the so-called “first night effect”. This refers to the tendency some of us have to sleep poorly the first night we are away from our normal bedroom environment. This research shows during the first night’s sleep away from home one hemisphere of our brain may have some limited response to external noises, while the other side does not. The research also showed on subsequent nights the unihemispheric response was reduced.
Many animals like dogs and cats are polyphasic sleepers, meaning they catch several naps throughout a 24-hour cycle, instead of having one unbroken phase of rest. Humans are typically monophasic sleepers, meaning our bodies and minds have over time adapted to sleep normally over one sustained time period.
Sleep is a dynamic process
Although characterized as a state of deep rest, neuroscientists using sophisticated measuring devices have shown there is much going on in our minds and bodies while we sleep.
We actually are in a dynamic state both physically and mentally while asleep. Electrical activity shifts within our brains, and various chemicals ebb and flow throughout our bodies. Key to this dynamic process are tiny structures in the brain that are sensitive to light and regulate our circadian rhythms.
Circadian rhythms – the cycles of day and night – control sleep to some degree in all living beings. The physiological mechanism that regulates sleep is sometimes referred to as the biological clock, and the surprisingly simple cue that synchronizes the internal biological clock to the environmental cycle is light.
Graphic: Yassine Mrabet
Idealized circadian rhythm of a person who rises early in the morning, eats lunch around noon, and goes to sleep around 10 p.m.
The specific neurological mechanisms by which light and dark regulate sleep are an ongoing subject of scientific study. The pineal gland appears to play an important role; this organ deep within the brain produces melatonin when it gets dark, and reduces melatonin levels when it gets light.
The body’s master biological clock is a tiny cell cluster in the brain called the suprachiasmatic nucleus, or SCN. The SCN responds to light, generating a “wake-up” cue to other structures located deep in the brain.
One such deep brain structure, the hypothalamus, is critical to sleep function by helping to regulate chemicals in our brains that promote both sleep and arousal.
Another structure called the thalamus helps us sleep by blocking input from our senses. The thalamus prevents us from waking up while sleeping – which in turn enables the brain to perform its critical function of reviewing and processing information from the day. The thalamus is impressively selective in what stimulation it blocks however; some people can sleep soundly through the roar of a freight train yet awaken to a baby’s cry.