Fasting is defined as a deliberate period of abstaining from all caloric intake. Any time spent not consuming food or drinks containing calories counts toward a fasting window. The hours spent asleep are the most common and easiest segment of time to include in any fasting protocol. The physiological state the body enters during sleep not only maintains the fast but often deepens it by preserving energy stores. Therefore, sleep is fully included in your total fasting time.
The Physiological Definition of Fasting
The fasted state, also known as the post-absorptive state, begins once the body has finished digesting and absorbing nutrients from the last meal, a process that typically takes three to four hours. During this period, blood glucose levels begin to fall, which signals the pancreas to decrease its secretion of the hormone insulin. A drop in insulin is the body’s cue to stop storing energy and start accessing its reserves.
With less insulin circulating, the body transitions from relying on readily available glucose to using stored fuel sources. The liver first releases glucose from its short-term storage form, known as glycogen, a process stimulated by the hormone glucagon. Glycogen stores are usually depleted within 12 to 24 hours, but the body begins its major metabolic shift much sooner.
Once the immediate glycogen reserves are running low, the body increases its reliance on stored body fat for energy. This shift involves the liver breaking down fatty acids into compounds called ketone bodies, which are then used as fuel by various organs, including the brain. The definition of fasting is based on this metabolic shift away from burning dietary sugar toward burning stored fat.
Why Sleep Maintains the Fasting State
The metabolic state achieved during the post-absorptive phase continues uninterrupted during sleep. While the body’s total energy expenditure decreases, it still requires a constant fuel supply to power essential functions like breathing, heart rate, and cellular repair. This ongoing energy demand ensures the body continues to draw from its stored reserves.
The brain’s glucose utilization is lowest during the deep, non-rapid eye movement (NREM) stages of sleep. Since glucose demand is reduced, the body conserves remaining glycogen and accelerates the reliance on fat stores. This reduced metabolic activity makes sleep a highly efficient time to deepen the fasted state.
Hormonal changes during sleep further support the fast, such as the pulsatile release of growth hormone during slow-wave sleep. Growth hormone helps maintain stable blood sugar levels by promoting lipolysis, the breakdown of fats. These integrated processes ensure the body continues to use internal fuel, maintaining the fasting window.
Structuring Your Fasting Window Around Sleep
Incorporating sleep into a fasting schedule is the most practical way to extend the fasting window without experiencing hunger. Schedule the final meal of the day several hours before bedtime. Finishing eating at least three hours before sleep ensures the body has completed digestion and entered the post-absorptive state before rest begins.
Delaying the first caloric intake of the day upon waking is the second half of this strategy. For example, a person finishing dinner at 7 p.m. and sleeping for eight hours accumulates 11 hours of fasting time. Waiting until 11 a.m. to eat extends the total fasting time to 16 hours, a duration used in many popular intermittent fasting protocols.
During the entire fasting window, only non-caloric substances should be consumed to avoid triggering an insulin response. Approved liquids include plain water, sparkling water, black coffee, and unsweetened herbal or green tea. Staying well-hydrated helps manage hunger signals and supports metabolic function while maintaining the fasted state.