In adults with overweight or obesity, consuming the first meal of the day after a 20-hour fast reduces the early-phase insulin response by 42% compared to earlier meal timing, without changing total insulin exposure over a 3-hour period, indicating a transient reduction in pancreatic beta-cell responsiveness to glucose.

After a long fast, the pancreas loses its readiness to release insulin quickly when food arrives. When eating happens late at night, the body's internal clock further suppresses insulin release, so blood sugar rises higher at first because not enough insulin is released right away. The pancreas still releases the same total amount of insulin later, but the delay means blood sugar stays high longer.