The brain’s prime directive to eat and
defend against the loss of fat emerged early in evolution, because just
about every creature that ever trotted, crawled, swam, or floated was
beset by the uncertainty of that next meal......
blood sugar, and weight gained per calorie consumed all ratchet up when
our sleep is disrupted and our circadian rhythms—the 24-hour cycle
responding to light and dark—thrown into disarray. All this is
compounded by stress, which decreases metabolism while increasing the
yen for high-calorie food......."
The hormone Leptin is produced and released by fat cells to signal satiation to inhibit appetite and influence metabolism.
The opposite effect is created by the hormone Ghrelin (anti Leptin).
findings that ghrelin is produced in mucosal endocrine cells of the
stomach and intestine, and that ghrelin is measurable in the general
circulation indicate its hormonal nature."
I believe the production of ghrelin is stimulated also by gut microbes
(especially those that thrive on high glycemic foods whenever these
sources of nutrition are minimized).