Time-restricted eating limits food to a window of several hours each day without specifying what is eaten. Two quite different mechanisms are proposed for it, and they are difficult to separate.
Metabolism runs on a daily cycle
Almost every tissue carries a molecular clock, and those clocks regulate the timing of processes including insulin sensitivity, digestive enzyme output and fat storage.
The system anticipates rather than reacts, preparing for food at times when food has historically arrived. This is why hunger appears at habitual mealtimes rather than at a fixed depletion point.
Insulin sensitivity is generally higher earlier in the day, meaning identical meals are handled differently depending on when they are eaten.
Light and food set different clocks
The master clock in the brain is set primarily by light reaching the eyes. Clocks in the liver, muscle and fat respond more strongly to when food arrives.
Eating at hours misaligned with the light cycle pushes these two sets of clocks apart, and that internal disagreement is the proposed source of metabolic strain.
Restricting food to a consistent daytime window is meant to pull the peripheral clocks back into line with the central one.
The fasting interval itself
A long overnight gap without food allows insulin to fall and keeps it low for an extended period, during which stored fat is drawn on more heavily.
Longer intervals are also associated with increased cellular housekeeping processes, though the evidence for this in humans on ordinary eating schedules is far less settled than popular accounts suggest.
Modern eating patterns often spread food across most of the waking day, so the overnight gap is shorter than the pattern the system evolved around.
Reduced intake is the confound
Removing several hours of eating opportunity usually reduces total intake, particularly by eliminating evening snacking, which is where a large share of discretionary calories sit.
Any weight change then has two candidate explanations, and studies that match calorie intake between groups find much smaller differences than studies that do not.
Which suggests that a meaningful part of the popular effect comes from eating less rather than from the timing itself.
Window placement matters more than length
Because sensitivity varies across the day, an early window and a late window of identical length are not equivalent, and research generally favours the earlier one.
A late window can conflict with the pattern the clocks expect, which may offset the benefit the restriction was meant to produce.
Anyone using insulin or other glucose-lowering medication, and anyone pregnant or with a history of disordered eating, needs medical guidance before changing meal timing.