Your sleep shows up at work: lessons from 2.1 million nights

Monday, 10:40, slide 14

It is 10:40 on a Monday. The quarterly update is on slide 14, and you have read the same line on the screen three times. You are at your desk, you are in the meeting, and very little is getting done.

Researchers have a word for this: presenteeism. It means being at work but working below par¹. It is harder to spot than a sick day, because nobody sees it.

In December 2025, a team in Japan put numbers on how much of it tracks back to the night before¹. They used sleep data from 79,048 working adults, collected by a smartphone app¹. Over 28 days that added up to more than 2.1 million nights¹. The phone's motion sensor logged each night, and each person then scored their own work over those same 28 days, from 0% to 100% of what they could do on a good day¹. The lost share is what the researchers counted¹.

Seven to eight hours, and not much more

The first finding is the one you would guess. Short nights went with more lost work. So did long nights. On a chart, the line makes a U¹.

The bottom of the U, where people lost the least, sat at an average of 7 to 8 hours of sleep a night¹. That lines up with the joint advice of the American Academy of Sleep Medicine and the Sleep Research Society: adults should sleep 7 or more hours a night, on a regular basis³. Below 7 hours, the same statement lists worse performance, more errors and more accidents³.

The twist is at the other end. The group the authors called "Long Sleepers" averaged about 8 hours 40 minutes a night, and they still lost more at work than the best group¹. More time in bed did not mean better days. The authors note that very long nights have been linked to sleep that does not leave you rested¹. So if you need 9 hours or more most nights, raise it with your doctor rather than aim for it.

Your weekend clock counts too

The app logged more than the length of each night. Taking a long time to fall asleep, lying awake in the night, a late body clock and a big gap between weekday and weekend sleep each went with more lost work¹.

The study calls that weekday and weekend gap social jetlag. It measured it as the difference between the middle of your sleep on free days and the middle of your sleep on workdays¹. Go to bed at 23:00 and wake at 07:00 in the week, then sleep 01:30 to 09:30 at the weekend, and the middle of your night moves by two and a half hours. Your body lives that shift every Monday, as if you had flown east.

When the researchers sorted people into five sleep profiles, this group stood out¹. The "Social Jetlaggers" lost about 3 more points of work on the 0 to 100 scale than the "Healthy Sleepers"¹. The group who took long to fall asleep and woke in the night came next, at about 2.5 points¹. A few points per person sounds small. Across a team of 40 people, week after week, it is a lot of slide 14s.

Why you may not feel it

An older lab study explains why this slips past us. In 2003, researchers kept 48 healthy adults on 4, 6 or 8 hours in bed for 14 days in a row². On 6 hours or less, mental test scores kept sliding, until they matched up to 2 full nights without any sleep².

The people in the study barely noticed. Their own sleepiness ratings rose at first, then flattened, while their results kept dropping². The authors wrote that they were "largely unaware" of it². So "I feel fine on six hours" is not proof that your work is fine on six hours.

Three habits, and one for employers

Pick a wake-up time and keep it, weekends included. A 2017 RAND report on the cost of short sleep lists a consistent wake-up time among the things a person can do for better sleep⁵. If you want a lie-in, we would keep it under an hour, so the middle of your night stays close to where it sits in the week.

Protect a 7 to 8 hour window. Count back from your wake-up time and put the bedtime in your calendar, like a meeting you do not move. The same RAND report also points to less time on screens before bed⁵.

Watch how long it takes you to drop off, not only how long you sleep. In the Japanese data, slow sleep onset and waking in the night mattered on their own¹. If that is you most nights for weeks, talk to your doctor.

For employers, there is a bill attached. RAND estimated that Japan loses about 600,000 working days a year to short sleep⁵. Seol and colleagues suggest that flexible hours may be one way to narrow the weekday and weekend gap, while stressing that this still needs testing¹.