A while back my brother was mentioning that one of his friends slept for only a few hours a night, and she didn’t understand the importance of sleep or how our body’s cellular clock works. Around that time I was doing some readings on the circadian rhythm, so figured to write a blog about the importance of sleep.

According to the National Sleep Foundation, around 47 million Americans do not get enough sleep. Such sleep deprivation results in injuries and decreased productivity, which can cost the country 18 billion dollars a year. Lack of sleep also contributes to 1.2 million car crashes a year, 20% of the annual total for all car crashes. A report in the NY Times says sleep deprivation causes over $400 billion in economic losses each year.

I will first start with the two different types of sleep. They are – non-REM sleep and REM sleep. REM sleep, short for Rapid Eye Movement sleep, is where most of our dreaming happens. Ever saw someone sleeping while their eyeballs are moving rapidly and wondered if and what they might be dreaming about? That’s REM sleep.

Stage 1 non-REM is light sleep lasting for 1 to 7 minutes where we can be woken up by someone calling our name. Our heartbeat, breathing, and eye movements start slowing down. Our muscles start relaxing and our brain waves also slow down.

Stage 2 non-REM sleep is another period of relatively light sleep lasting for 10 to 25 minutes. But awakenings are harder to occur in stage 2 than stage 1. Our muscles relax even further and our body temperature starts dropping. We spend more time in stage 2 non-REM sleep than any other stage. Although our brain waves continue to slow down, there are bursts of rapid electrical activity thought to protect the brain from awakening from sleep.

Stage 3 non-REM sleep is the deep sleep phase. This stage lasts about 20 to 40 minutes and is very difficult to wake someone up from. This is also the most restorative stage of sleep where the body heals itself from the stresses of the day.

After stage 3 non-REM sleep, the body transitions to a few minutes of stage 2 non-REM sleep before moving on to REM sleep, which first occurs around 70-80 minutes after falling asleep. Eyes move rapidly and brain wave activity increases. Breathing, heartbeat, and blood pressure also increase to near waking levels. This is also the stage where most of our dreaming occurs and our body becomes paralyzed so we don’t physically act out our dreams. As the night progresses, the amount of time we spend in stage 3 non-REM sleep decreases while our REM time increases. In the beginning part of the night a complete sleep cycle might be 70 to 90 minutes, gradually increasing to 90 to 120 minutes as the night goes on. As such, on a typical night we might get 5-6 full cycles of sleep.

What happens if we sleep too late at night? Our body’s sleep cycle spends more time in REM stage than in stage 3 non-REM deep sleep stage. That’s why if we don’t sleep enough, we don’t feel refreshed when we wake up.

Next I’ll talk about the biology of sleep. Sleep is controlled by two systems in our body – homeostatic sleep drive and the circadian rhythm.

Homeostatic sleep drive is a simple process to understand – the longer you have been awake, the more likely you are to fall asleep. While there are numerous biochemical processes that drive the homeostatic process, one of the mechanisms that would be familiar to us is the role of adenosine in sleep. Adenosine is released throughout the day by the energy giving molecule ATP. The more physical work we perform, the more ATP is broken down in muscles to produce Adenosine. And we all know how quickly we fall sleep after a physically tiring day! All the adenosine released from the breakdown of ATP binds to its receptors in the brain, releasing other chemicals that make us fall asleep. In contrast, caffeine competitively binds to the same receptors as adenosine, thereby blocking the actions of adenosine. There are other mechanisms of caffeine where it releases hormones that make us more alert in the morning, but by binding to adenosine’s receptors, caffeine plays an important role in negating the drowsiness of late afternoons.

The circadian rhythm is an internal clock that follows a 24-hour cycle. This rhythm affects every cell, tissue and organ in the body, and is controlled by a master clock. This master clock consists of a group of 20,000 neurons called the suprachiasmatic nucleus (SCN) and is located in the hypothalamus of the brain. As seen in figure 1, this region receives light directly from the eyes through the optic nerve. The SCN subsequently sends signals to other locations in the brain that control hormones and the body temperature. One of the locations the SCN transmits signals to is the pineal gland where melatonin is produced. This hormone is involved in wakefulness, and its production is switched off during the day when the SCN is sending signals to the pineal gland. In the absence of light, melatonin production is ramped up and we fall asleep. Because the circadian rhythm starts producing hormones that make us fall asleep as the evening wears on, eating late at night produces hormones that make us stay up later, and as such disrupting our circadian rhythm and the sleep cycle.

Figure 1: Light reaching the SCN via the optic nerve. Image from American Chemical Society

The rhythm and timing of the circadian rhythm also changes as we age. Melatonin is released later in the night in teens, causing them to sleep and wake up later. As we get older, melatonin is released earlier in the evening. Therefore, we can blame melatonin as to why old people fall asleep and wake up earlier than adolescents! The amount of sleep we need also varies with age. Newborn babies need nearly 16 hours of sleep. Young children and teens need 8 to 10 hours of sleep, while older adults don’t need more than 7-8 hours of sleep. For most people, 7 to 9 hours of sleep is most appropriate during the day. Averaging under 6 hours or over 9 hours has been associated with increased mortality. As with anything in life where balance is important, 7 to 8 hours of sleep at night, or 7 hours of sleep at night and 30 to 90 min nap during the day is the ideal way to get an adequate amount of sleep and feel well rested.

Figure 2: Recommended sleep durations

I am a fan of afternoon naps. Our circadian rhythm causes a dip in energy around 2 to 4 pm for most adults. This is when we feel tired after lunch. It isn’t just our food that makes us sleepy, but most of us are programmed to be sleepy in early afternoon. This is also a perfect time to nap because we spend more time on stage 1 and stage 2 of non-REM sleep, and some time in REM sleep. If we nap late in the evening, we tend to spend more time in stage 3 of non-REM sleep, or deep sleep. No wonder we wake up feeling groggy!

You may be wondering, how long is an appropriate nap time. That would be about 20 minutes, or 90 minutes. A 20-minute power nap would keep us in stage 1, or light sleep, and it improves our alertness, enhances our performance, and gives us better mood. And if you do have time, take a 90-minute nap which is a full sleep cycle. You would wake up feeling refreshed, and a full cycle nap boosts memory as well as our creativity. If you are feeling tired in the afternoon, don’t feel bad. Its just the body going through its cycle. And maybe tell your boss that you should take a nap for even better performance!

While researching for this topic, I came across sleep paralysis. I had experienced it numerous times but didn’t know what it was called. Probably about once a month I would wake up in the middle of the night with a sense of dread, and I would think a thief is in my room. I would try to move my body, scream, but my entire body would feel paralyzed. Well, what happens is that in REM sleep the body goes into a state of paralysis known as REM atonia. As mentioned above, REM sleep is where we have most of our dreams. Because the body is paralyzed in REM sleep,  we don’t act out on our dreams by kicking or lashing out. Ever had someone sleeping next to you kick you in the middle of the night? That’s them dreaming and their REM atonia not fully functional.

Scientists hypothesize that sleep paralysis occurs when transition from REM sleep to other sleep phases don’t go smoothly. The paralysis of the body in the REM spills over to other stages, and if we wake up in that stage we become aware of our paralysis. Why we feel scared and feel there is someone or ghost-like presence in our room, for that science doesn’t have an answer yet! But after learning this, the next time I wake up with sleep paralysis, hopefully I won’t feel scared that there is a burglar in my room!

Most of us have heard how lights from our electronic devices can negatively affect our sleep. When I turn on night mode on my iPhone or Windows computer, the screens turn into a yellowish tint. White LED light, or blue light in the electromagnetic spectrum, with its shorter wavelength and higher energy, have been implicated in suppressing melatonin and affecting our circadian rhythm. As seen in figure 3, the solar spectrum is mostly in the green-yellow wavelength, while candlelight as well as incandescent lightbulbs emit wavelengths mostly in the red region of the electromagnetic spectrum. This red light has the least power to shift the circadian rhythm and suppress melatonin. Researchers have found that exposure to blue light (white LED light) compared to green light (sunlight) can suppress melatonin twice as long. In contrast to white LED light, midday exposure to sunlight helps in releasing melatonin at night. Researchers have shown that office workers in windowless environments have lower sleep quality than those exposed to sunlight during the day. To get a good night’s sleep, it is ideal to avoid looking at our electronic devices two hours before bed, or wear blue-light blocking glasses or installing apps that filters blue/green wavelength at night and switching to dim, yellow/red lights. But blue light can have some useful purposes. It has been used in light therapy to boost attention, reaction time, mood, depression, as well as seasonal affective disorder.

Figure 3: White LED spectrum vs Solar spectrum. Image from ExtremeTech.com

This brings me back to the last topic, the actual health effects of sleep deprivation. Lack of sleep can cause a lot of problems such as obesity, diabetes, hypertension, and compromised immunological function. It also affects our ability to form memories. Lack of sleep or poor sleep increases the production of the stress hormone cortisol. It also increases release of insulin, a hormone that promotes fat storage. Insufficient sleep also causes lower levels of the hormone leptin, and higher levels of the hormone ghrelin. In combination, this causes food cravings and contributes to weight gain. We all know lack of sleep causes mood disorders such as crankiness and irritability. Lack of sleep can cause safety hazards at the workplace or road mishaps, among other things. A proper night’s sleep, as well as a full REM-cycle nap, helps in forming memories. It improves are mood and learning abilities.

Hopefully I have given some ideas about the importance of sleep. Going to sleep at an appropriate time, eating at least couple of hours before sleep, avoiding blue light before sleep, taking naps when possible, these are all important conditions when creating a healthy sleeping habit.