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 morning, eats lunch around noon, and goes to sleep about 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 cluster of cells in the brain called the suprachiasmatic nucleus, or SCN. The SCN responds to light, generating an alerting or “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.