Sleep & Circadian Rhythm Basics

What Is the Circadian Rhythm?

The circadian rhythm is a cycle of biological processes that are linked to the cyclical environmental cues of the day. This includes cues such as light exposure, the frequency and height of that light, temperature changes, food intake, exercise, etc. The human circadian rhythm lasts about 24 hours and 11 minutes, and resets itself daily when it is exposed to daylight, along with the aforementioned cues. Our brains have an internal “timekeeper” that releases hormones that trigger the body to carry our predictive daily management routines in response to these cues. Light, in particular, has the greatest effect on the regulation of our circadian rhythms. Suggesting that light exposure which represents the rise, zenith, and decline of the sun is crucial for ensuring a healthy physiological response to the daily and seasonal rhythms.

Most of the circadian signaling is transmitted through our eyes. When light enters the retinas and gets transmitted into the brain it stimulates the suprachiasmatic nucleus (SCN). The SCN is the master circadian clock in your body that regulates all the other biological rhythms and clocks. There are many different types of clocks and it’s thought that most organs, such as the liver, heart, and pancreas, actually have their own circadian clocks. That’s why all these different factors like sunlight, physical exercise, and eating affect the entire circadian rhythm of your body.

As hunter-gatherers, some people tended to sleep later making them better suited for guarding the camp later at night, and waking up later the next morning. However, that difference in sleep timing only differs by a few hours. Meaning, those night owls who claim that they naturally fall asleep at 2am in the morning are probably influenced by a bit more than their genetics. Humans are diurnal and should be sleeping when the sun is down, regardless of a person’s unhealthy sleeping behaviors. Staying up until the early hours of the morning tends to be the result of living in the modernized world, which presents us with many different circadian disruptors that often appear much more enticing than getting an early night. Factors such as shift work, playing video games, scrolling mindlessly on social media, and partying under fluorescent light are unnatural and incredibly bad for our health. By now, I’m sure we all know that though, right?

Which raises the issue that we as members of the modernized world are currently dealing with: inaccurate representations of the current time of day and season due to artificial exposure to blue light, eating high calorie foods too late in the day, high stress levels, etc., all appear to be influencing our health dramatically. Therefore, it is crucial that we receive accurate daytime cues that represent a specific period of the day, so that our internal systems can react accordingly. 

Biological Processes That Influence the Circadian Rhythm and That are also Modulated by Light Exposure: 

• Body temperature
• Pulse rate and blood pressure
• Reaction time and performance
• The production of melatonin (secreted by the pineal gland during hours of darkness and plays a crucial role in the sleep/wake cycle)
• The production of serotonin and dopamine (associated with alertness and inversely with sleepiness)
• The production of cortisol (especially active during the first 30min after waking)
• Intestinal activity

“Typical” Circadian Rhythm Timing

This varies with an individual’s chronotype, but only by a couple of hours at most. Extreme “night owls” seem to be environmentally conditioned rather than as a result of their genetics and can be retrained to fall within a more acceptable range. See the sleeping cheat sheet for a more in-depth analysis.

• 2400 – Midnight
• 0200 – Deepest sleep
• 0300 – Lowest blood pressure
• 0430 – Lowest body temperature
• 0600 – Morning
• 0645 – Sharpest rise in blood pressure
• 0730 – Melatonin secretion stops
• 0830 – Bowel movement likely
• 1000 – Highest alertness
• 1200 – Day
• 1430 – Best coordination skills
• 1530 – Fastest reaction time
• 1700 – Most efficient blood circulation and greatest muscle strength
• 1800 – Evening
• 1900 – Highest body temperature
• 2100 – Melatonin secretion starts
• 2230 – Bowel movements suppressed

Benefits of a Healthy Sleeping Routine

Sleep is the primary anabolic state during which the body replenishes its energy storages, regenerates tissues and produces proteins. Growth hormone and testosterone are released, to enhance muscle and neuronal growth. Not to mention improving adrenal gland function, liver detoxification, and immune system function while asleep.

While you sleep, your brain reorganizes neural networks and cleans up metabolic by-products, such as beta-amyloid plaques (a potential biomarker for the development of Alzheimer’s Disease). 

A good sleep contributes to better performance, awareness, mood, stress management, skin quality, sports performance, learning capacity, and general wellbeing. Everyone needs around 7-10 hours, depending on recovery/stress needs, except those who carry the DEC2 gene (who can survive on 2 hours less sleep than the average). Although, this gene is quite rare and I couldn’t imagine it meaning that the person actually “requires” less sleep. 

Negative Effects of a Disrupted Sleep Schedule

Staying up late releases more cortisol, which is known to increase the production of cell signaling molecules, such as cytokines. The common marker of inflammation, C-reactive protein (CRP), may also increase. Melatonin, which plays a role in initiating sleepiness in response to a reduction in sunlight (artificial blue light inhibits melatonin release) and environmental cues, also inhibits insulin production, suggesting that it may not be a great idea to eat a heavy meal before bed time, unless you would like to increase your risk of type 2 diabetes and obesity of course.  

Sleep deprivation has also been shown to raise systolic blood pressure and increase the consumption of fat and sugar-heavy foods. Creating an imbalance in appetite and blood sugar regulating hormones. Chronic sleep deprivation leads to insulin resistance, type 2 diabetes, depression, seasonal flu, cardiovascular diseases, weight gain, and increased chance of traffic incidents.

Sleep restriction may reduce T cells, making them more hyperactive, and levels of leukocytes, neutrophils, monocytes, natural killer cells, and pro-inflammatory compounds increase. You become less sensitive to insulin, which results in poor blood sugar regulation and weight gain. Levels of cortisol, epinephrine, norepinephrine, and dopamine increase, resulting in an overstimulated sympathetic nervous system, elevated heart rate, poor recovery processes such as muscle protein synthesis and digestion, and reduced amounts of IGF-1, growth hormone, and testosterone. You also suffer from psychological stress and have a higher risk of suffering from anxiety or depression.

If you’re competing at a high physical level, it may take you longer to fall asleep. Especially before a competition. Which is unfortunate as sleep restriction is a significant injury risk factor and is associated with reductions in neuromuscular control and proprioception. The increased levels of proinflammatory cytokines are what makes sleep-deprived athletes more likely to get respiratory infections. Long-term sleep restriction results in a progressive reduction of maximum and sub-maximum strength and can reduce the respiratory rate and time to exhaustion in maximum incremental exercise tests. Meaning, the more active you are, the more likely sleep deprivation will hurt you.

Without adequate sleep, reorganization of neural networks may not occur, our mind becomes cluttered and we run out of space for new memories, affecting nearly every system in your body.

Causing the following:

• Poor heat regulation
• A decline in immune function
• Increased levels of cortisol, catecholamines, interleukin, C-reactive protein, and other stress and inflammatory hormones
• Imbalances in appetite and blood sugar regulating hormones

How Much Sleep Do We Need?

Age, genetics, environment, and differences in daily physical and mental strain can all cause significant variations in the ideal amount of sleep. Most people should get no more than 9 hours, but no less than 7 hours sleep. However, physical activity affects sleep requirements and may lead to needing way more than usual.

Sleep Needs by Age:

• Newborns (0-3 months): 14-17 hours
• Infants (4-11 months): 12-15 hours
• Toddlers (1-2 years): 11-14 hours
• Preschoolers (3-5): 10-13 hours
• School-age children (6-13): 9-11 hours
• Teenagers (14-17): 8-10 hours
• Younger adults (18-25): 7-9 hours
• Adults (26-64): 7-9 hours
• Older adults (65+): 7-8 hours

Changes in Sleep Habits with Age

• Advanced sleep timing, in which you both go to bed and wake up earlier.
• Longer sleep onset latency, which means it takes longer to fall asleep.
• Shorter sleep duration.
• Increases in deep-sleep interruptions, such as frequent forays into light-sleep stages.
• More-fragile sleep, so that you are awakened more easily by noises and lights.
• Less slow-wave (deep) sleep. See “Sleep Stages” below for more information about this.
• Increases in lighter NREM sleep stages, which is less restorative than deep NREM sleep.
• Shorter and fewer sleep cycles.
• More time spent awake throughout the night.

Why Can’t We Rest Well? 

• Lack of sun exposure, reducing early morning and evening light cues.
• Artificial lights dominating light cues.
• Caffeine overload.
• Excessive stress and lack of perceived safety.
• Eating patterns and diets that are misaligned with the current season and timing.
• Conflicting modern cultural sleep hygiene.
• Disconnection from the earth.
• Pharmaceutical contraindications.
• Reduced movement.
• Unhealthy breathing patterns that increase stress levels.  

Sleep Stages

N1 – The first stage (theta waves, 4-8Hz): “Drowsy” sleep. This is the phase between wakefulness and light sleep. Your muscles are alert and active and your eyes may dart back and forth. The sleeper changes positions frequently, and is in a deep meditative state. If someone were to wake you up you might not feel like you were asleep. An EEG shows irregular oscillations of brain wave behavior. Your brain waves transition between beta and gamma waves with a frequency of 12-30Hz and 25-100Hz, which are typical of the waking state to slower, more synchronized alpha waves at a frequency of 8-13Hz and, finally, to 4-7Hz theta waves. You don’t usually dream in N1 and you might have micro-awakenings.

During this time (approx. 10 minutes) you may hear sounds and conversations, but you’re unlikely to respond. 

N2 – The second stage (sleep spindles, 11-16Hz): A period of light sleep, during which there is little movement and your breathing is quiet. Muscle activity declines and your awareness of your surroundings fades. More active brain activity and dreaming is possible. Getting enough stage 2 sleep improves your motor skills. Can be easily awoken. Primarily theta waves, but N2 sleep is distinguished by two things: sleep spindles, which are short bursts of 12-14Hz brain waves (sigma waves) lasting about half a second each, and K complexes, which are short, high voltage negative peaks followed by slower complexes called “positive complexes,” and finally, another negative K complex peak; each slower complex lasts one or two minutes. These sleep spindles and K complexes suppress your response to external stimuli and promote sleep-based memory consolidation and information processing. 45-50% of time is spent in N2.

Duration approx. 20-30min.

N3 – The third stage (delta waves, 0-8Hz): A period of deep sleep, where breathing is stable and EEG readings consist of slow delta waves. During N3, your awareness declines further. Your breathing rate, heart rate, brain temperature, blood pressure, and neural activity are at their lowest. You’re more likely to dream in N3 than during light sleep, possibly suffering from night terrors, sleep talking, or sleep walking. Memory consolidation and information processing also occur. Production of HGH begins, and the regenerative mechanisms of the body are activated. If woken from this state you would feel groggy and it would take 30 minutes to shake the sleep inertia. N3 makes up about 15-20% of total sleep time. Elderly people experience a shorter duration, by as much as 6min. 

Duration: 30-40min. 

R – REM Sleep (alpha and beta waves): The brain is awake, but the body is asleep. The muscles in the neck and body are paralyzed to prevent sleepwalking. Eyes move under the eyelids, and dreaming is at its peak. The typical adult has 4-5 REM stages a night. The first stage lasts about 10min, while subsequent stages are around 30min. REM is important for the regeneration of nerve cells. REM deprivation leads to irritability, fatigue, memory loss, and reduced capacity for concentration. Infants experience a lot of REM. On average 50% of the total 16 hours of sleep for an infant is REM.

REM sleep typically occupies about 20-25% of total sleep for adult humans, or about 1 and a half to 2 hours.

A typical adult’s 7–8-hour sleep moves through first->second->third->second. After this, the sleeper either wakes up or goes into REM. The cycle then repeats itself 4-5 times (90min). It is paramount to maximize deep sleep (N3) by going through at least 3 cycles. Getting enough sleep reorganizes one’s memory and improves one’s learning capacity. Later in the night, REM increases and deep delta sleep decreases.

Total sleep time

• Awake time: 1-5%
• REM sleep: 20-25%
• Light sleep (N1 + N2): 50-55%
• Deep sleep (N3): 15-20%

Going through a full cycle of the stages should take around 90 minutes and you should go through 4-5 of these sleep cycles during a 24-hour period. 

The Takeaway

In a similar vein to what we covered in the Breathing section, an effective sleep routine and sleep hygiene is necessary if we are to live happier and healthier lives. You just can’t neglect a good night’s sleep without suffering the consequences. On a positive note, it is an extraordinarily powerful tool for both stress recovery and physical performance of all aspects. If we choose to prioritize our blue light producing screen time and working late into the night over our sleep, we won’t make for old bones. If we weren’t so desperate to stay up and be entertained late into the night, our odds of acquiring modernized health problems, infection risk, and metabolic syndrome would drop immensely. 

I apologize for effectively scaremongering about the “sleep deprivation epidemic” but if we don’t take sleeping more seriously, we will continue to feel powerless and to reduce our dysfunction to bad luck due to a perceived lack of control. 

If you would like a sleep optimization planner that is derived from the recommendations of the pop culture neuroscientists, Matthew Walker and Andrew Huberman and biohackers from around the world, click here. 

Alternatively, if you want a more comprehensive cheat sheet on all things sleep, click here.