Circadian system Phase and Amplitude


Most people think of the circadian system ONLY in terms of phase – e.g., we would call a night owl “phase delayed” and an early bird “phase advanced”. This means that their internal clock is offset from the solar time by a certain amount.

So when you are a night owl who wants to wake up earlier, you want to “advance” your system by seeing more light in the morning and less at night. And the reverse for an early bird who wants to sleep in. As such, a lot of our users tend to be night owls. But you might think this is the end of the road for circadian topics – is your phase aligned with how you want it to be?


What is clear from experiments on shift workers and the elderly (and many rodent models) is that the overall circadian rhythm in the body can be suppressed. In some cases, it’s operating at a lower “amplitude” because light and dark are not clearly defined or not happening at the right times. For instance, people who spend time in natural lighting during the winter have an enormous melatonin amplitude thorughout the night (you can see melatonin in the blood lasting 14 hours), whereas most of us in the modern world are living in a “perpetual summer” and we have a relatively small melatonin duration in comparison to natural conditions.

We think some sleep problems (and some immunity and seasonal effects) depend to some degree on the “strength” of the circadian rhythm, not just whether it’s phase aligned. (Of course, a misaligned rhythm, e.g., in shift work, will almost always have a reduced amplitude, so they are somewhat related.)

Also, not all sleep trouble is circadian, so when we can, we look for ways to help people get to bed on time. It is probably true that we could do a lot of good here that’s completely unrelated to magic science and more to human behavior.

– Michael Herf, creator of f.lux

Leave a Reply

Your email address will not be published. Required fields are marked *