Red light and sleep: what a full-body study in footballers found

What the researchers did

This wasn't a trial set up in a lab. A university team in the US looked back at data a women's football team had already collected over one competitive season¹.

As part of their usual recovery routine, the players could book 20-minute sessions on a full-body light bed. It gave out red light at 660 nm and near-infrared at 850 nm, a dose of 14.4 J/cm² of each, on bare skin. They could have up to two sessions a week, never on days next to each other.

Every player wore a sleep-tracking ring each night, and a GPS tracker at every training session. Twelve players used the bed at least once, 106 sessions in all. The team compared each player's night before a session with their night after it, for the 93 sessions where the ring was worn on both nights¹.

What they found

The night after a session, the players slept less¹:

• Total sleep went from 7.93 hours to 7.29 hours on average, about 38 minutes less.
• REM sleep went from 1.07 hours to 0.87 hours, and light sleep from 4.62 to 4.30 hours.
• Deep sleep, time awake and sleep efficiency didn't clearly change.

At the same time, their average heart rate overnight was lower¹: 53.3 beats a minute against 54.3. Heart rate variability was a little higher, 103.7 ms against 100.5, but that difference was small enough that it could have been chance.

The drop in sleep held up when the researchers allowed for how hard each player had trained that day, and over the four days up to it. The authors suggest the players may have needed less sleep to recover, but say this hasn't been tested¹.

What it didn't test

The authors are open about most of these limits¹:

• No comparison group and no pretend session. Players knew when they'd had a session, so expectation can't be ruled out.
• A small group. 12 players, and some had far more sessions than others (8.5 each on average).
• One night at a time. It compared the night before and the night after, not weeks or months.
• A ring, not a sleep lab. Rings are good at sleep time and heart rate, less so at telling the stages of sleep apart.
• No performance or how they felt. It didn't measure training, tiredness or mood the next day.

The authors declared no conflicts of interest and no outside funding.

How this compares with a session at Sorted

The players lay on a full-body bed with two wavelengths for 20 minutes. Our full-body panels have seven wavelengths, from 480 to 1060 nm, and a red light session at the studio is 25 minutes, so the light and the dose aren't the same.

These were fit young athletes in a heavy training season. The study can't tell us whether the same would happen to anyone else, and it doesn't show that less sleep after a session is a good thing or a bad one. We don't claim either.