Do you lose more muscle on a diet when you sleep less?

Short answer

Yes. Cutting sleep to 5.5 hours on a diet made lean mass drop 60% more, while less fat came off. Sleeping about 1 hour less a day did not raise muscle loss, but less of the weight lost was fat.

Do you lose more muscle on a diet when you sleep less?
'Less sleep → less muscle building → less muscle' — does this formula hold?
On a diet, it is easy to cut sleep to fit in early workouts. Some say that when sleep runs short, more of the weight you lose is muscle. We checked studies that changed sleep and nothing else.
Same diet, less sleep
60% more lean mass was lost
For 14 days, 10 people ate the same reduced diet with either 8.5 or 5.5 hours in bed (no exercise). On 5.5 hours, lean mass (muscle and everything except fat) dropped 2.4 kg, 60% more than on 8.5 hours (1.5 kg). Fat lost actually fell, from 1.4 kg to 0.6 kg.
Why?
One sleepless night cut muscle building by 18%
The day after 13 people stayed up all night, the rate at which they built muscle protein after a meal was 18% lower than after a normal night. That day, the stress hormone cortisol rose 21% and testosterone fell 24%.
But exercise blocked it
On 4 hours a night for 5 days, exercisers held steady
When 24 people spent only 4 hours in bed each night for 5 days, their muscle-building rate fell to about 81% of the 8-hour group. Those on the same 4 hours who did 3 sessions of high-intensity interval exercise showed no difference from the 8-hour group.
What about 1 hour less a day?
Muscle loss didn't rise, but less fat came off
For 8 weeks, 36 dieters either slept normally or cut about 1 hour of sleep on 5 days a week and slept freely on the other 2. Weight, lean mass and fat lost were similar. But a smaller share of the weight lost was fat, and weekend catch-up sleep did not undo it.
Flip it: what if you sleep more?
People ate 270 kcal less a day
When 80 people who usually slept under 6.5 hours added about 1.2 hours of sleep a night, they ate 270 kcal less a day and lost weight, with no diet plan. Sleeping more alone made dieting easier.
Bottom line: big sleep cuts cost more muscle
Sleep is part of the diet plan
Cutting sleep to 5.5 hours on a diet led to 60% more lean mass loss and less fat loss. Cutting about 1 hour also meant less fat loss. Leaving around 8 hours for sleep, and keeping up training on short-sleep days, is enough.