In an early grant we co-wrote with our scientific advisor, Jaime Zeitzer, was this gem of a sentence: “A misalignment between the circadian clock and the need for sleep leads to fatigue and concomitant physical and mental impairment.” That may be easy for him to say, but we should probably unpack this statement and address why the circadian clock is fundamental to sleep quality.

The two process model of sleep regulation

In very simplistic terms, sleep is regulated by two body systems: sleep/wake homeostasis and the circadian biological clock.

Sleep/wake homeostasis is a fancy way of saying that we get tired when we have been awake for a long period of time— this is your basic need for sleep.  If this were to only process regulating our sleep patterns, then it would mean that we would be most alert as our day was starting out, and that the longer we were awake, the more we would feel like sleeping.

Source: National Institute for Occupational Safety and Health

But it’s obviously not as simple as this, as anyone who has struggled to fall asleep at night has experienced, and that is because our circadian biological clock regulates the timing of periods of sleepiness and wakefulness throughout the day. So if you’ve ever felt more sleepy in the early afternoon than 11 pm, that’s most likely a result of your circadian rhythm.

Normal circadian sleep rhythm. Sleep urge (circadian biological clock) is greatest at night with a small increase at mid-day. Sleep need (sleep homeostasis) increases throughout the waking hours and is replenished during sleep. source

The circadian system generates a “clock dependent alerting process during waking hours,” which basically means that your body produces hormones like cortisol that cause you to feel alert during certain times of the day, as set by your internal biological clock. Trying to sleep during these times leads to worse sleep quality, because you are essentially fighting your biological clock. For instance, if you are an extreme night owl (perhaps you even have Delayed Sleep Phase Syndrome), this means that trying to force yourself to go to sleep at 10 pm and wake up at 6 am is likely to result in poorer sleep quality than if you go to bed and wake up according to your own circadian rhythm, say going to bed at 2 am and waking up at 10 am.  Many, many teenagers have issues related to the conflict between their hormonally delayed circadian rhythm and biologically ill-advised early school start times (See here for more on adolescent sleep needs).

Adjusting Your Sleep-Cycle

One of the most effective ways to adjust your circadian rhythm is through the use of bright light, because light naturally regulates the circadian clock. As I wrote about in a previous post, light activates specialized non-vision forming cells in your eye called intrinsically photosensitive retinal ganglion cells (iPRGs), which then communicate with a region in your hypothalamus called the suprachiasmatic nucleus (SCN) to control the release of hormones like melatonin and cortisol.

Changing your circadian rhythm can be easy or extremely difficult, depending on your specific biology- it can be as simple as getting more sunlight at appropriate points during the day, or require a considerable amount of effort. Currently, all commercially available light-based approaches to regulating circadian rhythm involve the use of continuous bright light, which typically involves sitting in front of a bright light box for 1-3 hours per day. We are looking change that with technology that was discovered and clinically validated by Stanford researchers, and we are excited to offer a product that can not only collect sleep data, but can use this data to help shift your circadian cycle to optimize sleep quality.

So soon, you will be able to easily reset your circadian rhythm to meet the demands of your daily schedule!  Happy sleeping!