What is circadian sleep sufficiency?

Here’s a weird thing about shift workers: Their sleep duration doesn’t always predict how sleepy they feel. Which is weird because… shouldn’t it? After all, somebody who sleeps for a longer period of time should feel less sleepy than someone who sleeps only for a short period of time, right? 

This paradox isn’t actually that uncommon in sleep research. Often we encounter something called the “U-shaped curve,” which is, unsurprisingly, a curve in the shape of a U. All the time with sleep science, you see “Bad Thing” plotted versus “Sleep Duration” and see that the Bad Thing is high for both short sleep durations and long sleep durations. 

One way this could happen is if the short sleepers are experiencing Bad things because they’re not sleeping very long, while the long sleepers are experiencing Bad things and, because of that, they’re sleeping for a long time. This is what I tend to assume when I see a U-shape in sleep research, but it’s often not clear if these causative/correlative pathways are really what’s going on.

So something like that could be going on for shift workers. In this case, the Bad Thing is “feeling sleepy,” and the lack of a clear trend with sleep duration is due to the many complex and interacting factors surrounding how long you sleep and your overall health. 

Interesting research from a group at KAIST suggests another explanation. Their group developed a score called “circadian sleep sufficiency,” which describes, at the highest level, how “in sync” your actual sleep is with the times your body’s circadian clock wants you to sleep. One way to think about this score is that it’s more penalizing for short sleep times when you’re really dragging yourself out of bed, totally fighting your body’s clock, than it is for short sleep times when you woke up naturally because your body’s clock lightly shoved you out of sleep. In both cases, your sleep duration could be short, but in one, it’s shortened against the will of your body, while in another, it’s shortened because your body couldn’t sleep any longer at that time.

While sleep duration didn’t predict sleepiness in their dataset, circadian sleep sufficiency did. This suggests that there might be a way to warn shift workers when they run the risk of being extra sleepy due to an early alarm that has them fighting their body’s timekeeping system. This isn’t a way for shift workers to figure out how to get by with less sleep—nearly all of them are already running critically low on sleep as it is—but rather to help them understand when cutting their sleep short is most likely to pay off in a wave of sleepiness and performance impairment. 

Our app, Shift, features scores of our own that we’ve developed to capture how in sync your sleep, exercise, and meals are with your body’s internal rhythms. Try them out for yourselves by applying to Shift’s early access program or scheduling a pilot today.