tl;dr: During a diet
- With 5.5 hours of sleep per night: more than 70 percent of weight loss came from lean body mass—muscle, not fat.
- With 8.5 hours of sleep per night: well over 50 percent of weight loss came from fat while muscle was preserved
I wrote “The Importance of Sleep for Fat Loss” vs Weight Loss because I posit that 99.9% of the time, when someone says they want to “lose weight”, what they are actually saying is: “I want to lose fat and look more trim and toned.”
For example, one way a friend of mine says this is: “I don’t want fatty muscle, I want skinny muscle.”
In other words, people don’t want to lose muscle (they actually might want more muscle), they only want to lose fat.
So where does sleep come in? The amount of sleep one gets directly impacts the amount of fat loss vs muscle loss when losing weight:
Researchers had a group of overweight men and women stay in a medical center for an entire fortnight. One group of individuals were given just five and a half hours’ time in bed, while the other group were offered eight and a half hours’ time in bed. Although weight loss occurred under both conditions, the type of weight loss came from very different sources. When given just five and a half hours of sleep opportunity, more than 70 percent of the pounds lost came from lean body mass—muscle, not fat. Switch to the group offered eight and a half hours’ time in bed each night and a far more desirable outcome was observed, with well over 50 percent of weight loss coming from fat while preserving muscle. When you are not getting enough sleep, the body becomes especially stingy about giving up fat. Instead, muscle mass is depleted while fat is retained.Chapter 8, Why We Sleep
The scariest part about this is that the impact of sleep on obesity can start very early in life:
Three-year-olds sleeping just ten and a half hours or less have a 45 percent increased risk of being obese by age seven than those who get twelve hours of sleep a night. To set our children on a pathway of ill health this early in life by way of sleep neglect is a travesty.Chapter 8, Why We Sleep
In summary, sleep is always important but it is especially important when it comes to weight loss: “Lean and toned is unlikely to be the outcome of dieting when you are cutting sleep short. The latter is counterproductive of the former.“