We’ve already looked at the Multi-Ethnic Study of Atherosclerosis (MESA) dataset—an absolute treasure trove of sleep data, available from the NSRR at sleepdata.org—once, through the lens of sleep duration. But what about other dimensions of sleep health? After all, sleep regularity may be just as important as sleep duration in a number of contexts.
We once again teamed up with Ryan Rezai, a data scientist and student at the University of Waterloo, to visualize some MESA data. Once again, all the plots below were made by Ryan to highlight some intriguing trends in the MESA dataset. As always, we think there’s a lot of value in looking for pictures that can help you grapple with the complexity of multifaceted, complex phenomena like sleep.
Let’s start with the basics:
First we need to define sleep irregularity. We can do this in a number of ways. In the plots below, we’ve defined it in the ways MESA does—as either the standard deviation in total sleep duration (sd24hrsleep5) or the standard deviation in bed time (sdinbedtime5).
So how does sleep irregularity, as defined above, relate to Epworth Sleepiness Scale self-reports (ESS) in the dataset? Like this:
Looking at this, I feel pretty confident that there’s a trend here! As sleep irregularity increases, so does ESS, up until you get up to pretty profound variability in sleep regularity (180 minutes = 3 hrs standard deviation—you might be a shift worker at that point).
It’s even more remarkable when you look back at sleep duration and ESS (see last blog):
Not only does sleep regularity seem to have a clearer trend with ESS than sleep duration, it also seems to have a slightly higher amplitude effect, as shown on the y-axis. Right off the bat, this is a clue that we might be wanting to pay more attention to sleep regularity when we talk about the experience of sleepiness.
If we look at both bedtime irregularity and sleep duration simultaneously, we can notice something else interesting:
Namely, that even for people sleeping quite a long time (e.g. 500 minutes), greater bedtime irregularity is linked to greater feelings of subjective sleepiness.
What might be going on here?
We know a person’s subjective experience of sleepiness doesn’t always line up with how restricted their sleep has actually been. For instance, people on four hours of sleep a night tend to get worse and worse at reaction time tests, while their subjective sleepiness grows for a while but eventually levels off. Maybe irregularity makes people more reliably aware of just how sleepy and impaired they are because their irregularity means they’re more likely to be awake during periods of time when melatonin is at a high concentration in their body. Or maybe irregularity is having a dampening effect on their body’s circadian rhythms, making them more likely to feel exhausted and flat. There’s plenty of work to be done here in the future, but I’ll take off my Hat of Speculation now.
Beyond sleepiness
These 3-D sleep plots can be used to visualize more than just ESS. Take, for instance, this plot of total apneas over the course of the night as a function of sleep duration and sleep regularity:
Ok, wow! That’s a clear picture, albeit perhaps not a surprising one in some ways. After all, you’d expect a longer duration of sleep to mean more opportunities for apneas. That said, it is interesting how, once you get above about 300 minutes of sleep (5 hours) or so, holding sleep duration fixed and increasing irregularity seems to correlate with increased apneas.
Something similar appears to hold for sleep irregularity, sleep duration, and apneas per hour, with more sleep irregularity linked to a higher rate of apneas—at least when you ignore people sleeping around 200 minutes a night (which, to be clear, is probably not that many people):
Sleep irregularity and heart rate
Lastly, we might be interested in how sleep irregularity correlates with heart rate. After all, recent research has shown the risk of a cardiovascular event is more than twice as high in irregular sleepers as it is in regular sleepers. When we look at the irregularity in sleep duration, it sure looks like there might be something going on with average heart rate and irregularity in how long you sleep:
This trend also seems to hold when you look at the correlation between bedtime irregularity and average heart rate:
For both of these plots, the standard error is smallest between 0 and 180 minutes of standard deviation in sleep irregularity—and like we noted earlier, three hours standard deviation in sleep irregularity is a lot! (What’s going on when the standard deviation of bedtime irregularity is around 300 minutes? I sure as heck don’t know. But since that’s a standard deviation of five hours in sleep irregularity, odds are good that that’s not the typical sleeper.)
On the whole, it seems pretty clear: People who care about their overall sleep health shouldn’t sleep on sleep regularity.
With thanks to these resources:
The Multi-Ethnic Study of Atherosclerosis (MESA) Sleep Ancillary study was funded by NIH-NHLBI Association of Sleep Disorders with Cardiovascular Health Across Ethnic Groups (RO1 HL098433). MESA is supported by NHLBI funded contracts HHSN268201500003I, N01-HC-95159, N01-HC-95160, N01-HC-95161, N01-HC-95162, N01-HC-95163, N01-HC-95164, N01-HC-95165, N01-HC-95166, N01-HC-95167, N01-HC-95168 and N01-HC-95169 from the National Heart, Lung, and Blood Institute, and by cooperative agreements UL1-TR-000040, UL1-TR-001079, and UL1-TR-001420 funded by NCATS. The National Sleep Research Resource was supported by the National Heart, Lung, and Blood Institute (R24 HL114473, 75N92019R002).
