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Worry keeps us awake. Forty percent of American adults say they have difficulty falling asleep at least a few times each month. The most common reason is an inability to stop thinking about...whatever it is you can’t stop thinking about: A project for work. Unpaid bills. That thing you said that you wish you hadn’t. We call it “whirring” in my house.

A new study in the Journal of Experimental Psychology suggests an easy and effective solution: Write in a journal for five minutes before bed. But critically, what helps most is not writing about what you accomplished during the day, but writing out your to-do list for tomorrow.

In the study of 57 young adults, researchers from Baylor University and Emory University found that writing to-do lists, rather than writing about completed tasks, helped people fall asleep an average of nine minutes faster — in about 16 minutes versus 25. That’s an effect size comparable to recent pharmaceutical clinical trials in which people taking sleep aids have fallen asleep nine to 10 minutes faster than usual, reports lead author Michael Scullin, a psychological scientist and sleep researcher at Baylor: “This seems to be a quick little thing people can do in the evening, not to fall asleep in two minutes, but to fall asleep faster than they probably would have otherwise.”

Previous research has connected writing and lessening of anxiety, and even writing and better sleep, but Scullin’s study is the first to use the gold standard of sleep measurement, EEG, to determine exactly how much faster people fall asleep — and it’s the first to specify the content of the writing.

The findings hit unusually close to home for me. One of my Christmas presents was a “Night Thoughts” journal. (It says so right on the cover!) At bedtime, I’ve started diligently writing about what I got done and how I feel about it. Oops.

I recently interviewed Scullin about what he thinks might be at work here.

Why does writing at bedtime help you get to sleep?

Throughout the day, we have all these things cycling through our head. Some of them seem to continue to cycle. There’s something about the act of writing — physically writing something on paper — that tends to offload it a little bit, or help us hit the pause button on it. The outcome seems to be that you decrease cognitive arousal, and that you decrease rumination and worry. If you decrease those two things, it makes sense that you’re going to fall asleep faster, because having stuff on your mind is one of the main barriers to falling asleep at night.

Why would a to-do list be more helpful than a list of completed tasks?

When you have a task that’s unfinished, it’s on your mind more than any task you have completed. If you test people’s for things that were unfinished versus things that were completed, people remember the things that were unfinished a lot better. It seems that unfinished tasks rest at what we call a heightened level of cognitive activation. We think that’s the key ingredient. With our day-to-day lives and work schedule, unfinished tasks pile on one another and create this cognitive activation that’s difficult to set aside — unless, of course, you write about it.

How might this apply more broadly?

I’m curious about the generalizability to clinical groups, especially individuals with a specific subtype of in which they’re fine on total sleep time; they just have a lot of difficulty falling asleep. They already have some good treatments. Can we make those current treatments even better? They sometimes incorporate some writing activities, like writing down what you’re about during the day. None of them are incorporating any to-do lists into bedtime writing. The question is: Would adding that benefit these patients?

Did to-do lists improve sleep in other ways?

Not to statistically significant levels. Sleep onset latency was our primary measurement, but we had one other effect that was trending toward significance. The number of times that people woke up in the middle of the night seemed to be lower. And anecdotally, people say, Yeah, when I’ve got a lot on my mind, I tend to wake up earlier. So maybe for some people, a to-do list could help you sleep a little bit longer. I’ll be interested to see whether that turns up again in future studies.

The study only looked at one night. Do you think the effect can be sustained?

We haven’t tested that. It could be yes, because each night you’ve got this big to-do list. But it’s also true that the to-do list fluctuates, and how much you accomplish during the day also feeds into that. So maybe it’s going to be most effective on the nights when you have a whole lot of stuff to do, and it’s more likely to be eating at you if you don’t write things down.

Scullin plans to study these open questions. I plan to write a to-do list in that Night Thoughts journal tonight.