Humans are hard-wired to need sleep. The countdown begins from the moment you open your eyes: The longer you’re awake, the sleepier you become until you reach the point where you’re ready to turn in for the night. That’s due to sleep pressure. But there’s a second major player: your internal clock, which is synchronized to the light-dark rhythms of a 24-hour day.
Together, these form the two-process system. Known as Process S (for sleep pressure) and Process C (for circadian rhythm), they influence when you feel sleepy and when you feel alert. Here’s what to know about these two crucial aspects and how they function.
Sleep pressure
Sleep pressure, or the urge to sleep, slowly builds up the longer you’re awake. The increasing sleepiness you feel is due to the gradual build-up of the neurotransmitter adenosine.
The urge to sleep is at its lowest when you first wake up, hitting its peak right before you drop off to sleep. Then, as you slumber, adenosine levels gradually return to baseline, resetting so the process can begin again. (This is also known as the sleep homeostatic process, reflecting the ongoing rebalancing between sleep and wakefulness.)
This pressure builds more quickly in young children, which is why they typically need daytime naps.
Adults are generally awake for about 16 hours before the pressure to sleep kicks in, although many power through to stay awake even longer (as reflected in the most recent results gathered by the Centers for Disease Control and Prevention, showing that close to 38% of adults generally get less than seven hours of sleep at night).
Despite the “sleep when I’m dead” bravado, eventually, we all need to sleep. One of the most famous records for staying awake took place in 1963 when 17-year-old Randy Gardner remained awake for 264 hours — a feat that was only possible because he had researchers prodding him awake whenever he attempted to drift off. In similar experiments, people have experienced hallucinations and paranoia; the Guinness Book of World Records no longer tracks the longest time without sleep because of the risks involved.
Even in less dramatic cases, the overwhelming urge to sleep can cause people to nod off suddenly. This is especially dangerous if the sleeping person happens to be behind the wheel: According to the most recent estimates, drowsy driving was a factor in about 17% of fatal crashes.
How to relieve sleep pressure
Naps: The best antidote to sleepiness is, of course, sleep. While a complete reset requires a full night’s sleep, you can get a partial reprieve — enough for a temporary recharge — by taking a nap. Even a brief nap of about 20 minutes can restore alertness. When naps are too long, though, the opposite can happen: You’ll fall more deeply asleep, which can leave you feeling groggy (a phenomenon known as sleep inertia).
The other issue with naps? If they’re too long, they can reduce your sleep pressure too effectively, so you’re not feeling as sleepy at bedtime.
Caffeine: If you can’t carve out time for a nap, a jolt of caffeine can also help you feel more alert. Your coffee or energy drink isn’t reducing your sleep pressure, though; it’s only masking it.
That’s because caffeine binds to the adenosine receptors in the brain and temporarily blocks adenosine from reaching them. When the effects of the caffeine wear off, you’ll once again feel sleepy from the elevated levels of adenosine (which has continued to build despite being held back by the caffeine).
The circadian cycle
The second process, Process C, refers to your internal clock and how it’s influenced by external cues of light and darkness.
Though this internal clock is slightly longer than the planet’s rotation, at about 24 ¼ hours, our internal clock is synchronized to the 24-hour cycle of day and night by regular exposure to light and darkness, the primary zeitgeber, or time cue, for our circadian clock.
Exposure to light begins from the moment we wake and helps cue us to feel alert, thanks to the blue light that’s part of the daylight spectrum. This regular exposure to outdoor light, which contains far more blue light than indoor light, plays a key role in keeping our circadian clock synchronized.
Most adults are indoors for much of the day, depriving us of the potent cue we would get from being outside. Without this strong daytime light signal to calibrate our internal clock, we’re much more affected by the light we get during the evening hours from overhead lighting and from back-lit electronic devices, which all emit blue light.
Regardless of the time of day, blue-light exposure activates cells in the retina that then release melanopsin, which “basically sends a ‘daylight’ input to the clock,” explains Michael Grandner, Ph.D., director of the Sleep and Health Research Program at the University of Arizona.
“As a society,” he adds, “we’re much more susceptible to light at night because we’re not flooding our system during the day.”
There’s a second important zeitgeber that’s also hard at work regulating our internal clock: melatonin, a hormone produced by the pineal gland deep inside the brain. Under normal conditions, melatonin is released a few hours before bedtime, prompting us to start feeling sleepy.
However, this timing can be disrupted by inopportune signals to our internal clock that it’s still daytime, such as when we’re exposed to blue light from our phones and other screens in the evening. When our brains receive this daylight input, the melanopsin that’s produced suppresses the timing for the release of melatonin, delaying our sense of sleepiness.
In addition to the larger cycle of daytime and nighttime, smaller peaks and valleys of alertness also occur during our waking hours.
“There’s a natural dip in the circadian rhythm after lunch,” explains Shelby Harris, a licensed clinical psychologist and board-certified behavioral sleep medicine specialist. While this postprandial dip in alertness is a main driver of early afternoon drowsiness, big midday meals and large amounts of carbohydrates may also contribute.
Circadian fluctuations are also at work during the early evening when a small peak offers a temporary boost of energy.
What happens when these two processes aren’t aligned?
While these two processes are independent of one another, they generally operate in a way that’s aligned. The result: You feel sleepy at night and alert during the day.
Sometimes, however, these two processes can be out of sync, which can spell trouble.
Up all night
People who work overnight shifts face this most acutely: They’re awake when their circadian clock would normally be cueing them to sleep. While there are various strategies to help cope with the fallout, shift workers — who make up about 25% of the workforce — have to override their natural urge for nighttime slumber and then compensate by sleeping during the day.
In many cases, they’re not getting enough sleep due to a confluence of factors, including daytime noise and light, as well as family and life demands. Beyond the various risks associated with being sleep-deprived, however, this circadian misalignment increases their likelihood of developing a wide range of health issues, including cardiovascular disease, stroke, and chronic pain. Sleep issues for those facing overnight work shifts are so common that there’s an official diagnosis: shift work sleep disorder.
The perils of jet lag and social jetlag
Traveling by plane across various time zones is another frequent source of circadian misalignment. The symptoms of jet lag are generally temporary, dissipating as we slowly adjust to the new time zone — a process that’s greatly helped by daytime light exposure at our destination.
There’s a similar dynamic, known as social jetlag when our sleep/wake schedules aren’t consistent. While it may not seem like a big deal to sleep late on weekends to try to make up for lost sleep during the week, the consequences are quite significant: Chronic inconsistent sleep increases mortality risk and contributes to metabolic syndrome and cardiovascular disease, among other risks.
Changes over the lifespan
Both processes in the two-process sleep model — your sleep pressure and circadian clock — evolve over your lifetime, starting in infancy.
While infants — unable to discern yet between day and night — sleep and wake at various times around the clock, their circadian rhythm generally stabilizes during the first year so that they sleep primarily at night. Babies and young children also build up sleep pressure more quickly (as reflected by their need to nap).
The next major circadian shift takes place during adolescence, when kids’ natural sleep-wake cycle moves later, driven by the later release of melatonin (the hormone prompting sleepiness). This later timing peaks at about age 20 before gradually shifting back.
Then, as we age, this timing moves even earlier, with over-65-year-olds having earlier bedtimes and wake times than younger adults.
Sleep pressure, which rises more slowly in adults than young children, slows even further as we age. At the same time, the various mechanisms that keep the circadian rhythm aligned to day/night cues no longer function as well. In addition to having lower melatonin levels, older adults likely have less sensitivity to the external cues that help keep the circadian clock on track.
Focusing on healthy sleep
Regardless of age, maintaining a focus on healthy sleep habits can help minimize issues.
Among the areas to focus on:
- Maintaining a consistent sleep schedule: Keeping irregular hours is even worse than not getting enough sleep, prompting a recent consensus statement by the National Sleep Foundation emphasizing the importance of sleep consistency.
- Harnessing the power of daylight: As noted, exposure to light during the day plays a key role in keeping our central circadian clock in sync. Aim to get outside, particularly in the morning, each day.
- Making sleep a priority: Follow best practices for sleep hygiene, including having a winddown routine to help ease the transition to sleep.
- Seeking help if needed: If you’re experiencing insomnia, are regularly waking feeling unrefreshed, or have other ongoing issues, it may be time to consult a health care provider.
