What is the Sleep–Wake Cycle?
The sleep-wake cycle refers to the pattern of time we spend awake and asleep every 24 hours. This pattern is one of the body’s many circadian rhythms (1) and is species-specific. For humans, the 24-hour clock is divided between approximately eight hours of sleep and 16 hours of wakefulness. The most significant role of the sleep-wake cycle is to consolidate sleep (2) during the night, helping you stay awake during the day.
In addition to the sleep-wake cycle, other circadian rhythms exist to regulate numerous bodily functions (3) that rise and fall over a 24-hour pattern. Some of these functions include hormone production, core body temperature, energy levels, and appetite.
Circadian rhythms work alongside homeostatic sleep pressure to help guide the sleep-wake cycle. Homeostatic sleep pressure (4), or the need to sleep, is at its lowest after a night of restful sleep and slowly builds up throughout the day. Strenuous physical or mental work, long hours spent awake, or a compromised immune system can all result in increased sleep pressure. A healthy sleep-wake cycle is achieved when your body’s circadian rhythm and homeostatic sleep pressure are synchronized.
What Happens to the Brain During the Sleep-Wake Cycle?
Every cell in the body contains its own biological clock, which is synchronized by the suprachiasmatic nucleus (SCN), located in the brain. Certain genes produce proteins that increase overnight and fade during the day. These changes activate feelings of wakefulness and sleepiness, which can affect when you sleep and how alert you are when awake.
The most important external influence affecting the body's internal clock is sunlight. When the eye’s optic nerve senses daylight, the SCN releases chemical signals, such as the hormone cortisol (5), and neurotransmitters (6), such as norepinephrine and serotonin, to help keep the brain alert and awake.
Over the course of the day, another chemical, called adenosine (7), accumulates in the bloodstream and eventually makes you feel tired. The strongest dip in rhythms of wakefulness — also called the afternoon slump — generally comes between 2 p.m. and 5 p.m., when the desire to nap is strongest. As the day goes on and daylight fades, the brain begins to release the hormone melatonin, which helps prepare for sleep.
Once the brain is asleep, an important cleaning process takes place. The glymphatic system (8) named after the brain’s glia cells, clears out waste present in the brain, such as proteins. Although more research is needed, the glymphatic system's increased activity during sleep could help explain why sleep helps heal damage after strokes and traumatic brain injuries.
What Impacts Circadian Rhythms?
Circadian rhythms are impacted by both internal and external factors (9). Circadian rhythm-related sleep disorders that originate within the body are called intrinsic circadian sleep disorders. Those that originate from outside the body are often referred to as circadian rhythm disorders.
As infants and children age, the sleep-wake schedule shifts, and the needed amount of sleep (10) tapers off. Infants, for example, need between 12 to 16 hours of sleep each day, while school-aged children need nine to 12 hours. Adults should aim to get between seven to nine hours of restful sleep each night.
Many teenagers experience a sleep-phase delay (11), in which their brain doesn’t start producing melatonin until late in the evening. Late nights paired with early school wake-up times can take a significant toll on mental health and make it hard to stay focused.
Blue Light Exposure
Blue light waves (12) are found in fluorescent and LED lights and electronic screens, such as phones, laptops, and television. Exposure to blue light waves at times when the brain should be producing melatonin, which is in the evening for most people, can halt the process and ultimately shift the circadian rhythm. As a result, blue light exposure can make falling asleep more difficult.
Jet Lag and Daylight Saving Time
Crossing two or more time zones can result in jet lag (13), a sleep-wake disorder that occurs when the body's internal clock is aligned with the timezone of origin and doesn’t match that of the new location.
Daylight saving time (14) also disturbs the sleep-wake cycle, but to a lesser degree than jet lag, since the time shift only amounts to one hour. That said, the shift associated with daylight saving time comes with major consequences. Every year, there are increased health problems, mood changes, and even car crashes as a result of this sleep-wake schedule disruption.
Shift work with irregular hours may make it necessary for workers to be awake at night and sleep during the day. As a result, shift workers must function at a high level at times that don’t match the natural daylight cycle and their own circadian rhythms (15) This contradiction can not only lead to a sleep-wake disorder, but to various other negative health consequences as well.
Caffeine is one of the most common stimulants used in the world (16). Caffeine succeeds in making people feel more awake and alert by blocking the adenosine receptors in the brain. Without caffeine, adenosine promotes feelings of sleepiness as it builds up in the body over the day.
What Disorders Occur When Circadian Rhythms Are Off?
Circadian rhythm sleep-wake disorders are disturbances of wakefulness and sleep. They often cause difficulty falling asleep, staying asleep, and sleeping at typical times, resulting in chronic fatigue. Multiple sleep-wake disorders (17) exist, including:
- Delayed Sleep-Wake Phase Disorder: When sleep and wake times are pushed to at least two hours later than what is desired, a person has a delayed sleep-wake phase. For example, someone with this disorder might want to fall asleep by midnight, but be unable to sleep until 2 a.m. A delayed sleep-wake phase can make adhering to a normal schedule difficult.
- Advanced Sleep-Wake Phase Disorder: People with this disorder experience the opposite problem that people with delayed sleep-wake disorder deal with. They find themselves falling asleep and waking up earlier than they would like.
- Shift Work Disorder: This disorder is unique to people who must work nights or on a changing schedule. People with shift work disorder find they have trouble sleeping when they'd like because their unusual schedule throws off their circadian rhythm.
- Irregular Sleep-Wake Disorder: In this disorder, there is no regulation of sleep-wake times. As a result, people with this disorder often sleep across several shorter periods throughout the 24-hour day.
- Non-24-Hour Sleep-Wake Disorder: Commonly occurring in people with blindness, non-24-hour sleep-wake disorder develops when a person's circadian rhythm doesn't conform to the 24-hour day. Instead, their circadian rhythm is slightly longer, so each night they go to sleep and wake up a bit later than they did the day before.
- Jet Lag Disorder: Jet lag happens after a person crosses two or more time zones in a relatively short period of time, usually while flying. Their circadian rhythm remains aligned with their original location, so they have trouble falling asleep and waking up at times that align with the destination location.
How Can I Improve My Sleep-Wake Cycle?
The sleep-wake cycle can be improved through better sleep hygiene. Sleep hygiene (18) is the sum of healthy habits surrounding a regular sleep schedule. Elements of sleep hygiene include:
- Waking up and going to sleep at consistent times
- Exposing yourself to natural light during the daytime
- Eating a healthy diet and exercising regularly
- Avoiding electronics that emit blue light waves before bedtime
- Refraining from caffeine too close to bedtime
- Keeping the bedroom dark, cool, and quiet
Sleep is a critical aspect of overall human health. Making sleep hygiene a priority is a good idea. However, it’s important to remember that certain sleep and circadian rhythm disorders may not be improved or managed without the help of a doctor.
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