Being tired changes how your body uses and stores energy. Your body might be permanently trying to save up for bad times (because the times are always bad) and so you're getting fat even if you eat very little.
it's not something I have ever been made aware of, and I don't understand how it could be the case.
I don't have anything specific to link - it's just my own personal experience and what the doctor said.
In my own case, I had to eat much more than I theoretically should because my body wasn't able to process the food as well as it should (less than 50% of usual efficiency). And yet I was fat - not obese, but definitely not thin. The body redirected some of my food to storage even though I didn't have enough energy (glucose etc) as measured by blood screening. And it didn't use the reserves properly when I stopped eating.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7898726/
> Effects of moderate sleep restriction during 8-week calorie restriction on lipoprotein particles and glucose metabolism
Because the conclusion starts with,
> In this study, 8 weeks of calorie restriction with and without sleep restriction of less than 90 minutes per day on 5 days with 2 days of ad libitum sleep per week, resulted in similar degrees of weight loss
Let me be clear that sleep deprivation doesn’t seem to be good for your circulating lipoprotien levels or insulin sensitivity based on that study, but they didn’t gain weight.