More research concludes that only 4 nights of sleep impairment makes the body less physiologically capable of fat loss: http://www.jlr.org/content/early/2019/09/04/jlr.P094375
Hormones run the show. Some anachronistic luddites still talk about calories (some of which DOES matter). But it’s all about hormones. People with optimal hormone patterns eat more and move less. People with suboptimal hormone patterns can starve as much as they want and exercise as much as they can, and they’ll never fix the fundamental problem.
For years I’ve tried to buck this trend. I have iron will. I know nutrition science backward and forward. I run crazy experiments. I’m a health and fitness professional. But in long stretches of extreme high work load and impaired sleep, my body CANNOT get lean. It doesn’t matter how great my program compliance is, how tight my nutritional and workout program are, I cannot supersede this biological law.
No one can.
Trust me. I believe in hard work. I know what it is to outwork and outwill every single person around you. But at some point, suboptimal stress management (including bad sleep) come a-knockin. You can’t outwork too much work. A handful of my peers still cling to this idea, talking about how they soldiered through high-stress periods of their lives and still got super-shredded or muscular. The same guys gave themselves autoimmune diseases in the process, having to have portions of their colons removed or some other severe issue. The irony is totally lost on them. "Unrelated," they say. No. Not unrelated. I am in agreement that you can try to push through and make the calories paradigm work; but there is a monumental cost.
This is where some of the influencers and famous motivational speakers come up totally bereft of wisdom. Yeah. I get it. A lot of people could benefit from working harder. But not ALL people.
Lack of sleep doesn’t scale. It has high associations with autoimmune disorders, rampant inflammation, neurodegenerative diseases, cancers, and cardiovascular risks. So, even if someone curtails sleep in order to achieve one thing (let’s say it’s a body composition goal), I have supreme skepticism about other areas of his health especially over the long term.
A gain somewhere incurs a cost elsewhere. You can’t out-diet or out-exercise bad sleep.