Twenty five years ago, a friend trying to win a weight loss bet slept in winter temps in order to convert normal fat into ‘brown’ fat. It, along with diet, modest working out, and sleep modification worked as he lost a very impressive amount of weight in a short time. He had to keep it off for a year to win the bet, and as far as I know he never gained it back. He credited the brown fat. YMMV.
What kind of sleep modification? Polyphasic?