Everyone has an innate biological way of keeping time—an internal master “clock” that coordinates the processes and changes in your body.
But unlike an actual clock, our master clocks are composed of about 20,000 neurons located within the suprachiasmatic nucleus, or SCN, a part of the hypothalamus region of the brain. This master clock is responsible for many of the human body’s internal processes, such as appetite, blood pressure, alertness, and performance.
There are four main types of biological rhythms controlled by the master clock:
- The shortest, ultradian rhythms, occur multiple times within a 24-hour cycle and repeat throughout the day, such as alertness or hunger cues.
- Infradian rhythms occur on a longer-term basis, such as the menstrual cycle.
- Diurnal rhythms are rhythms synced with night and day, such as waking up with the light and getting tired as night time comes around.
- The final type of biological rhythm is one that many people are more familiar with—the circadian rhythm.
What Is a Circadian Rhythm?
The circadian rhythm is a biological rhythm that exists in many species (including humans and certain types of plants, fungi, and insects) and coordinates physiological processes on a 24-hour cycle. Although many people only associate the circadian rhythm with sleep, the circadian rhythm is also responsible for coordinating hormone release, impacting your eating habits, and regulating body temperature.
Cortisol and melatonin are the primary hormones controlling circadian rhythms. Cortisol, a hormone that controls alertness and contributes to human stress and immune responses, is produced in higher quantities in the morning. Cortisol production decreases throughout the day, while melatonin production begins to ramp up at night to prepare the body for sleep.
The varying levels of melatonin and cortisol (among other hormones) throughout a 24-hour cycle change how your body reacts to the surrounding environment. For example, blood cortisol levels are highest in the morning, around 7:00 am, right around the same time melatonin production stops.
As the levels of these hormones change throughout the day, you experience periods of higher alertness or better ability to think clearly and make decisions quickly. Towards bedtime, melatonin production starts, reaching a peak between 1:00 am and 4:00 am.
The changes in melatonin and cortisol levels correlate with sleep cycles—a typical person usually has between four and six sleep cycles a night, lasting 90 minutes each. The earlier sleep cycles are generally shorter, while later sleep cycles tend to last longer.
Disruptions to Circadian Rhythms
Your circadian rhythms can be thrown out of sync in a multitude of ways. Those who have experienced jet lag are likely well aware of how a seemingly short flight can throw off their sleep cycle for quite a few days after. The same is seen in people who work shift work or odd hours.
Other common disruptors are genetic disorders and light from electronic devices late at night that interrupt the body’s normal production of hormones that contribute to the maintenance of a circadian rhythm.
Although humans now have more devices that produce artificial light than they did even 20 years ago, work and school start times have stayed the same. As a result, more individuals struggle to fall asleep, wake up, or maintain a consistent sleep cycle.
Circadian rhythm disorders, also known as sleep-wake cycle disorders, are caused when someone’s environment doesn’t match their natural sleep-wake cycle due to their work schedule, frequent travel, or other lifestyle habits. Someone with a circadian rhythm disruption may have problems falling asleep or staying awake at the desired times. Different circadian rhythm disorders can be diagnosed and treated depending on the root cause of the disruption and someone’s natural sleep cycle.
Sleeping in Cycles
Each night, you undergo four stages of sleep, cycling through all four stages about four to six times. The first three phases are non-REM (non-rapid eye movement sleep), while the fourth phase is REM sleep (rapid eye movement).
Scientists categorize the different sleep stages based on brainwave frequencies and electroencephalogram (EEG), which test the brain’s electrical activity. Different distinguishing frequencies and bodily behaviors mark each stage.