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 in humans that dictates when you are most alert and when you are ready for sleep.

The circadian rhythm acts as each person’s internal clock. It operates 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.

While the general timeline of the circadian rhythm is similar for most people, there can be significant variation in each individual’s circadian rhythms. It is possible to change your circadian rhythm through a few different mechanisms, like consistently changing your bedtime and wake time, exposure to sunlight, 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. Accordingly, they will be most alert at 1pm, instead of 10am.

To fall asleep earlier and wake up earlier than your current sleep pattern, it is necessary to change your circadian rhythm. 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 bed- and wake-times, 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

Your body has two main processes that dictate your alertness, how long you have been awake, called your homeostatic sleep drive, 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 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. These external cues are known as “Zeitgebers” or time keepers. When your body is exposed to sunshine, it causes a chemical reaction 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. Similarly, it often takes longer to fall asleep if you have just been exposed to bright light than if you were in a dimly lit room. 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. If you wake up before the sun rises, using a sunlamp to imitate the sun’s rays can be an effective tool to help combat sleepiness, without needing to use caffeine.

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 an hour before periods of tiredness can also help to keep you feeling fresh. By tracking your own periods of peak alertness and sleepiness, you can ascertain your own personal circadian rhythm. 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.


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 10% of the population are larks, 20% are night owls, and intermediate, 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 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 the 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
• Exercise at the same time every day
• Stimulate yourself with social interactions at the same time every day