The Circadian Rhythm

The 24 Hour Cycle That Dictates Energy Levels

Overview of the Circadian in Humans

Every living animal has a circadian rhythm in one fashion or another. In humans, the circadian rhythm is a 24 hour cycle that dictates when you are most alert and when you are ready for sleep. A typical circadian rhythm in humans is one where peak alertness is around 2-3 hours after awakening and 8-9 hours after awakening, and where fatigue is most likely at around 3 AM, if you wake up like most people do at around 7-9 AM in the morning. However, this rhythm is impacted by many factors and it can be shifted. A major takeaway is that a health circadian rhythm can get you a deeper night of sleep, where the most important thing is regularity. Everyone can get deep, more regenerative, sleep when they give their body the cues it needs when it needs it.  SleepSpace seamlessly measures your circadian rhythm by integrating with the consensus sleep diary, various wearables, and the SleepSpace Smart Bed

The hormones that get released based on the circadian rhythm and how it impacts various aspects of the body including alertness, reaction time, and even bowl movements

The circadian rhythm acts as each person’s internal clock. It operates in humans on a 24-hour cycle and affects various biological processes throughout the day, such as sleepiness, peak exercise potential, body temperature, and blood pressure. Many people find themselves particularly tired at around 2am to 4am, and drowsy in the hours after lunch, at around 2pm to 4pm. Many also experience periods of peak alertness, usually occurring between 9am to 11am and 7pm to 9pm. These cyclical periods of sleepiness and alertness are due to the circadian rhythm. Humans have an overall circadian rhythm and then each organ actually has its own rhythm as well, which operates within the overarching circadian rhythm.

While the general timeline of the circadian rhythm is similar for most people, there can be significant variation in each individual’s circadian component. It is possible to change your circadian rhythm through a few different mechanisms, like consistently changing your bedtime and wake time, exposure to sunlight, relaxation exercises, and temperature changes. When your bedtime and wake-up time is consistently later than average, then your circadian rhythm is shifted to later hours. In other words, instead of becoming tired at 10pm, a person with late sleep patterns might not become tired until 1 in the morning.

Perhaps you have a goal to wake later or fall asleep earlier, then you can take steps to shift your circadian rhythm to meet this goal, which we outline below. It can be difficult to adjust sleep schedules because the circadian rhythm affects tiredness levels. If you are naturally tired in the early morning hours and fall asleep at 2am, it will be frustrating and likely futile to try to sleep at 10pm. Instead, try moving your bedtime back by 20-30 minutes gradually. As your circadian rhythm adjusts to your new bedtime and wake-time, you will find yourself becoming sleepy at the time you desire. Similarly, slowly adjusting your wake time can be more effective than a sudden shift. When wake times and bedtimes are consistent, your circadian rhythm will help you fall asleep and wake up more easily, as you will feel tired before bed and alert as you wake up.

How the Circadian Rhythm Impacts Alertness

A graphic articulation of a healthy circadian rhythm of a morning lark

Your body has two main processes that dictate your alertness, how long you have been awake, called your homeostatic sleep drive, which is largely impacted by deep sleep, and the 24-hour circadian rhythm. These two processes, combine to form the two process model of sleep and a typical energy level like what is articulated in the above graph. Notice how there are peaks and valleys and the overall reduction in alertness until bedtime. If you are a morning lark, this rhythm will be shifted earlier in the day, whereas if you are a night owl it will be shifted later.

Light Exposure and the Circadian Rhythm

Circadian components are affected by sleep schedules, but they are also affected by your environment, particularly, exposure to sunlight, but also when you eat, exercise, and even when you socialize. These external cues are known as “Zeitgebers” or time keepers. When your body is exposed to sunshine, it causes a chemical reaction through photoreceptors in your eyes that send a signal to your suprachiasmatic nucleus that makes your body feel more alert. By opening your blinds and turning on the lights as soon as you wake up, you will shake off morning grogginess more quickly than if you leave the blinds closed. Another hack if you want to counteract morning grogginess - try splashing some cold water on your face to start revving up your core body temperature.

The other side of light exposure is that it often takes longer to fall asleep if you have just been exposed to bright light close to bedtime (especially blue light). By adjusting light exposure, you can help your body to feel alert or sleepy, depending on if you want to wake up or fall asleep. Getting blackout blinds is one of the best solutions to start controlling your light exposure. Also limit sunglass usage during the day to ensure you get the light you need to set your rhythm. If you wake up before the sun rises, or live in a place without much sunlight, using a sunlamp to imitate the sun’s rays can be an effective tool to help combat sleepiness, without needing to use caffeine. However, getting direct sun exposure, not through a window, but directly outside is probably the best way to entrench a healthier circadian rhythm. This will also help you fall asleep, stay asleep, and get a deeper night.

Napping, Caffeine, and Exercise Based on the Circadian Rhythm

Napping during the day can help to alleviate the mid-day sleepiness that comes with the circadian rhythm. Short naps of 20-30 minutes during periods of tiredness can help keep you focused and alert throughout the day. Alternatively, drinking caffeine can also help you counteract that afternoon dip. For the best effect with caffeine in terms of addressing that afternoon dip, try not to drink it right in the morning, but around 60-90 minutes after you wake up. Then try to stop drinking caffeine at around 2 pm - 3 pm (based on a typical sleep schedule of waking up between 7-9 am. Also, some people just don't like taking naps, and that can be perfectly healthy as well. There are a few different styles of sleeping that can lead to a healthy circadian rhythm, many of which are impacted by societal pressures and responsibilities. By tracking your own periods of peak alertness and sleepiness, you can ascertain your own personal circadian rhythm. There are even genetic tests you can take that tell you if you have genes that make you more likely to be a morning lark, night owl, or as we call it, a flexible octopus.

Take our sleep quiz to discover your sleep animal. Knowing your unique energy rhythm means that you can better schedule your daily activities. If you always feel tired at 4pm, then you can schedule a run or a nap before this period to help reduce your feelings of tiredness. If you are at your most alert at 10am, then scheduling important meetings at this time can ensure that you will be alert and at you best. Generally, most people find that it is better to do analytical work in the morning and more creative work in the late afternoon or night.


Some people are morning people or ‘larks’ – others are evening people or ‘owls.’ This tendency to have a peak alertness either earlier or later in the day is known as chronobiology. Your unique chronobiology is influenced by your genetic makeup and environmental cues called zeitgebers. About 25% of the population are larks, 10% are night owls, and the rest of us are shiftable, with a circadian rhythm that can be shifted by external cues that you give to your body. Everyone can shift their circadian rhythm to some degree with these zeitgebers. The evolutionary basis for this relates to always ensuring that a member of the tribe is awake to protect the camp. Unfortunately our modern society was forged out of industrialism in a way that is unjustly preferential towards early rises, with strict morning work times.

For an in-depth understanding of chronobiology and sleep inertia check out an interview Dr. Daniel Gartenberg gave to Quartz entitled “Why eight hours a night isn’t enough, according to a leading sleep scientist.”

Zeitgebers: Time Givers

Thankfully, we can impact the expression of our genes through our environment in order to create a healthier circadian rhythm. Zeitgebers are environmental cues like sunlight, exercise, and eating that impact your circadian rhythm and energy levels throughout the day. Temperature and even social interactions can also create environmental cues that tell your body when to be alert. These external cues reset the internal body clock, also known as your circadian rhythm. Naturally occurring sunlight is the strongest zeitgeber because of its impact on hormones like melatonin. Even when our eyes are closed, photoreceptors in the eye can detect sunlight. These photoreceptors respond to sunlight by inhibiting melatonin. Due to the fact that we have become indoor creatures, people are often out of touch with their circadian rhythm. This happens when you do not entrench your zeitgebers within your daily schedule.

Try to do the following things at a similar time every day:

  • • Sunlight in the morning when you wake up

    • Similar bedtimes and wake times

    • Eat at consistent times in the day and not too close to night. If you want to be a morning lark, eat and exercise more in the morning, if you want to be a night owl, shift these behaviors to be more in the later part of the day

    • Exercise at the same time every day

    • Raise your body temperature during the day, either with cardiovascular exercise or a sauna

    • Stimulate yourself with social interactions at the same time every day