How much does strength drop when you sleep a little less?

Short answer

Hardly at all when sleep was cut to 6 hours or less. In those studies, strength was 2.77% lower, but this did not count as a difference. The drop came when short sleep went on for 3 nights: thigh strength in 18 people fell by about 8%.

How much does strength drop when you sleep a little less?
'1-2 hours less sleep → less recovery → less strength': how true is this formula?
It is common to hear that the weights will not move on a day when you are short on sleep. We checked how much strength actually drops after sleeping a little less than usual, not after an all-nighter, and under which conditions it drops more.
Cutting sleep to 6 hours or less
Strength was 2.77% lower, and this did not count as a difference
In a 2022 analysis that pooled 69 papers with 959 people, 25 studies with 289 people measured strength. When sleep was cut to 6 hours or less, strength was 2.77% lower, but this did not count as a difference. Going to bed late gave an even smaller 1.16%.
What about skipping sleep entirely?
A similar 3.00%, but counted as a real drop
In the same analysis, staying up all night lowered strength by 3.00%, which counted as a difference. Combining shortened sleep and all-nighters, strength was 2.85% lower overall. That is like someone who lifts 100 kg lifting about 97 kg.
It depended on when and what was measured
Evening 4.58%, morning 1.78%, no difference for the upper body
After sleep loss, strength was 4.58% lower when measured in the evening and 1.78% lower in the morning. With shortened sleep and a morning test, it was 0.43%, with no difference. Lower-body strength dropped 3.42%, while the upper body's 1.63% did not count as a difference.
Things changed when short sleep went on for days
5 hours for 3 nights: thigh strength down about 8%
When 18 people slept only 5 hours for 3 nights in a row, the maximum holding strength of the front thigh muscle fell by about 8%. In a study where 8 people slept only 3 hours for 3 nights in a row, maximum bench press, leg press and deadlift dropped, while the biceps curl did not. Repetitions with lighter weights got worse from the second night.
After 9 nights, quality dropped before quantity
Total workload down less than 1%, lower-body speed down up to 15%
10 people who do strength training slept only 5 hours a night for 9 nights. The total weight lifted fell by less than 1%, but bar speed in lower-body compound lifts was up to 15% slower, while the upper body was not. The same workout felt 11% harder.
Summary: a day of slightly shorter sleep barely showed
The drop came when short sleep went on for days
On days with sleep cut to 6 hours or less, strength barely dropped, and strength measured in the morning showed no difference. When short sleep went on for 3 or more nights, thigh strength and lower-body bar speed dropped, and after 3 nights of 3 hours, the bench press dropped too. On short-sleep days, training in the morning meant a smaller drop.