The Sleep-Wake Cycle is Critical for Health

Sleep is a part of the body’s natural 24-h clock, which is called the circadian rhythm. The circadian rhythm regulates the body’s sleep-wake cycle by using information from the environment to help our bodies wake up in the morning and go to sleep at night. The cycle depends on messenger chemicals called hormones produced in the body, and it is in a delicate balance. When it is in proper balance, the circadian rhythm allows us to stay awake and alert during the day, and we have enough energy to complete our tasks. A balanced circadian rhythm also allows us to go to sleep on time at night, recover from tiredness, eliminate cellular waste products that have built up, and repair any minor damage to our muscles that we developed during our daytime activities—making us feel refreshed and ready to go again in the morning.

We depend on light in the morning and darkness at night to help the body’s clock continue turning on schedule, regulating the body’s processes. When it gets dark outside, the absence of light sends a signal to the brain that it is time to start the sleep cycle part of the 24-h clock. The brain interprets darkness as a message saying “there is no need to stay awake and alert”, and a pea-sized gland in the brain called the pineal gland begins to release a hormone called melatonin into the bloodstream. Melatonin is essential for the sleep-wake cycle and it helps promote good-quality sleep so that your body recovers and feels rested in the morning.

The sleep cycle has four stages: N1, N2, N3, and rapid eye movement (REM) (Figure 1). All the stages are important. During N1–N3, growth occurs, the muscles and the immune system are repaired, and the body and mind recover from tiredness. In the REM stage, the brain is highly active just like when we are awake, and this stage is important for memory development, learning, and dreaming, as well as for decreasing the level of a type of cellular waste product called free radicals [1]. When free radicals are present in high concentrations, they can damage both the body and brain cells.

For healthy, restful sleep, we must go through several sleep cycles each night. Interruption or a decrease in the number of completed cycles can lead to poor-quality sleep, leading to poor cognitive performance, decreased recovery, and incomplete free radical elimination. Repeated interruption or decreased time sleeping can lead to a condition called oxidative stress, in which the level of free radicals gets dangerously high. This happens because bodily processes that normally slow down while we sleep keep running instead, like a car that never shuts off and keeps producing exhaust. Elevated levels of free radicals can decrease health and cognitive performance [1]. Interestingly, melatonin can help to neutralize excess free radicals in the body, so it is believed to have a strong protective effect against oxidative stress [2].

How do Light and Darkness Regulate the Sleep-Wake Cycle?

All pathways that control essential functions in the body exist in balance with other pathways that produce the opposite effects. This way, the pathways can regulate each other. This is the case with the hormones melatonin and cortisol (Figure 2). Cortisol is a stress hormone released by the adrenal glands, which sit atop the kidneys, and it is responsible for creating the state of being awake and alert. In the morning, as the light from the sun enters our eyes, a signal is sent to the brain that acts like a wake-up alarm. The release of melatonin stops and the level of cortisol in the bloodstream rises, causing us to feel awake and alert. This process aligns the body’s clock to the time of day.

However, the same wake-up alarm effect happens when we use technology at night. The light from electronic devices enters our eyes, sending a signal to the brain that it should still be awake, alert, and on duty. One type of light produced by most devices, called blue light, is especially good at preventing melatonin release. The sun emits more blue light in the morning as it rises than at night when it sets—so our bodies are designed to respond to blue light by “waking up” [3].

By preventing melatonin from being released when it should be, the brain is tricked into thinking it is still daytime even though it is dark outside. Therefore, we do not fall asleep when we should—a condition called sleep latency—because the brain is receiving the message that it should be awake, alert, and functioning. Sleep latency puts the circadian rhythm off balance, which can lead to a number of problems.

Tech at Night Affects Mental Health

Just like our battery-powered devices, our bodies need to “recharge” to function at their highest capacity. If our bodies’ “batteries” run down and we do not stop to recharge, our bodies will not run as efficiently nor function at their best (Figure 3A). Furthermore, the bodily systems for regulating mood depend on sleep. When sleep latency happens, other pathways that regulate cortisol levels will also be affected [4]. In this case, people can develop mood disorders related to elevated cortisol, including depression and other mental illnesses [3, 4]. Scientists have discovered that people who work night shift jobs are 40% more likely to develop depression [4], probably due to the interruption of the circadian rhythm and general lack of sleep.

Protecting Yourself from Sleep Problems

Even if you currently use your devices at night and experience the negative effects lack of sleep can have, there are ways to help reverse the effects and fix any imbalance in your circadian rhythm. While scientists found that depriving rats of REM sleep for 96 h increased free radicals and oxidative stress in the rats’ brains, these effects were reversed after the rats had some restorative sleep [1]. This means that, even if you feel unwell after a few nights of poor sleep, you can restore your circadian rhythm, reduce your levels of free radicals and oxidative stress, and get yourself back to functioning normally—simply by getting some good-quality sleep.

Furthermore, limiting or completely eliminating the use of electronic devices before bedtime can prevent interruption of the circadian rhythm, which will prevent sleep latency and help the natural sleep-wake cycle (Figure 3B). Switching the lights in your home to lower the amount of blue light you encounter leading up to bedtime can help, too. Red light has been shown to improve sleep quality, and yellow and orange lights have little to no effect on the circadian rhythm and may increase melatonin production.

Finally, if you must use technology at night, using a pair of blue light-blocking glasses and turning on “nighttime mode” in the evening can help to decrease the amount of blue light your devices give off. This will limit the signals to your brain that prevent melatonin release and cause sleep latency, which can help you get a better night’s sleep [3, 4].

Overall, sleep is one of the most important contributors to our mental and physical health and cognitive function. Taking care to place our devices aside about an hour before bedtime makes a world of difference in the sleep quality we have each night as well as our overall physical and mental health. As a result, our circadian rhythm will be in balance and we will be able to fall asleep and wake up around the same times every day. Through making these efforts, we can ensure we get regular, good-quality, and uninterrupted sleep to stay healthy and productive.

Glossary

Circadian Rhythm: ↑ The body’s clock, which is in a delicate balance and is aligned with daytime and nighttime.

Melatonin: ↑ A hormone produced and released by the pineal gland, to begin the sleep stages of the sleep-wake cycle. Levels of melatonin are increased in the absence of light.

Free Radicals: ↑ Cellular waste products that, when present at high levels, can damage cells and lead to oxidative stress.

Cognitive Performance: ↑ The ability to think, reason, solve problems, pay attention, and remember things.

Oxidative Stress: ↑ A condition in which high levels of free radicals cause damage to the brain and body, which can lead to disease.

Cortisol: ↑ A stress hormone released from the adrenal glands, responsible for the wake stage of the sleep-wake cycle. Cortisol levels increase when light enters our eyes.

Sleep Latency: ↑ The delay of the start of the sleep stages of the sleep-wake cycle, which can occur when we use devices at night.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

All figures were produced by DH and NC, 2023.

Image credits Figure 1: https://www.freepik.com/premium-vector/lungs-great-design-any-purposes-cartoon-illustration-simple-vector-illustration_28611922.htm, https://www.istockphoto.com/vector/red-heart-vector-gm842899542-137687935. https://www.iconfinder.com/icons/2329949/arm_bicep_cartoon_hand_muscle_muscular_sport_icon, https://www.vecteezy.com/vector-art/2889592-cartoon-man-dre ams-of-money-house-and-luxury-car-while-sleeping-vector-illustration, https://www.vectorstock.com/royalty-free-vector/thermometer-cartoon-icon-on-white-background-vector-7571739. Image credit Figure 2: https://www.rawpixel.com/search/brain%20drawing?page=1&path=_topics&sort=curated. Image credits Figure 3: https://www.etsy.com/listing/885717259/75-bold-cartoon-style-ios-14-icons-cute, https://easydrawingguides.com/coloring-pages/cartoon-bed/, https://www.istockphoto.com/vector/funny-mischievous-cartoon-brain-character-gm1221591976-358143481, https://www.carstickers.com/products/stickers/anatomy-stickers/brain-stickers/marketplace/brain-depressive-cartoon-sad-tired-brain-sticker/.