At 24 hours without food, your body has largely exhausted its stored sugar and is shifting to burn fat for fuel. This transition triggers a cascade of hormonal changes, including a dramatic spike in growth hormone and, counterintuitively, a drop in the hunger hormone that made the first several hours so uncomfortable. Here’s what’s actually happening inside your body at this milestone.
Your Fuel Source Has Switched
Your body stores sugar (glycogen) in your liver and muscles as its quick-access energy supply. When you eat normally, those stores get topped off regularly. Once you stop eating, your body draws down that glycogen over roughly 18 to 24 hours. By the 24-hour mark, your liver glycogen is depleted or nearly so, and your body has started breaking down fat stores for energy instead.
This fat breakdown produces molecules called ketones, which your brain and muscles can use as fuel. Ketone levels in the blood typically become detectable after about 21 hours without food (or closer to 17.5 hours if you exercise during the fast). At 24 hours you’re in the early stages of ketosis, with ketone levels just beginning to rise. They won’t peak for days. During longer fasts, ketones can safely climb to around 4 mmol/L by day 12, but at the one-day mark you’re just entering this metabolic state.
Growth Hormone Surges
One of the most striking changes at 24 hours is a sharp increase in human growth hormone (HGH). This hormone helps preserve muscle tissue and accelerates fat breakdown, which makes biological sense: when food is scarce, your body protects its lean mass and mobilizes fat instead.
The size of this spike depends on your baseline levels. A randomized controlled trial found that people who started with low HGH saw a median increase of 1,225% during a 24-hour fast, with some individuals experiencing increases as high as 20,000%. Those who already had higher baseline levels saw a more modest median bump of about 50%. Across the board, a separate study found HGH increased roughly 5-fold in men and 14-fold in women over a single day of water-only fasting. Women tend to see a larger response, likely because their baseline levels differ from men’s.
Hunger Peaks, Then Fades
If you’ve made it to 24 hours, you’ve probably noticed something surprising: the intense hunger from earlier in the day has eased. This isn’t just willpower or distraction. It’s hormonal.
Ghrelin, the hormone that signals hunger, normally spikes before your usual meal times and drops after you eat. You might expect it to climb relentlessly during a fast, but research shows the opposite. In a study of healthy women, mean ghrelin levels actually decreased during fasting compared to a normal eating day, dropping from about 441 to 360 pg/mL. Peak ghrelin levels fell too, from 484 to 376 pg/mL. The timing of the ghrelin peak also shifted dramatically, moving from early morning (around 4:15 a.m.) to late afternoon (around 5:15 p.m.). So hunger doesn’t keep building indefinitely. It comes in waves tied to your habitual eating schedule, and those waves tend to get smaller as the fast continues.
Your Metabolic Rate Holds Steady (or Rises)
A common worry is that skipping food for a full day will slow your metabolism. At 24 hours, this isn’t what happens. Short-term fasting actually tends to maintain or slightly increase resting energy expenditure, driven in part by a rise in norepinephrine, a stress hormone that keeps your body alert and active. One study published in the American Journal of Clinical Nutrition found that resting energy expenditure rose from about 4.0 to 4.5 kilojoules per minute over the first few days of fasting.
This makes evolutionary sense. If your ancestors’ metabolism crashed the moment food ran out, they’d have been too sluggish to go find more. The metabolic slowdown people associate with dieting typically comes from prolonged calorie restriction over weeks, not a single day without food.
Autophagy May Be Starting
Autophagy is your body’s cellular recycling program. Cells break down damaged or dysfunctional components and repurpose the raw materials. It’s one of the most talked-about benefits of fasting, but the timeline in humans is still uncertain. Animal studies suggest autophagy may begin somewhere between 24 and 48 hours of fasting, according to Cleveland Clinic. At the 24-hour mark, you’re potentially at the front edge of this process, though researchers haven’t yet pinpointed the exact trigger point in humans.
Electrolytes Stay Stable (For Now)
At 24 hours, your electrolyte balance is largely intact. Your kidneys actually tend to hold onto sodium during the first day of a fast. Research published in the Journal of Clinical Investigation found that sodium excretion sometimes decreases slightly on day one, resulting in a small positive sodium balance. The significant sodium losses that people associate with fasting, sometimes called the “natriuresis of fasting,” don’t typically begin until around 48 hours in. After that point, the kidneys start flushing sodium at an accelerated rate, which is why longer fasts require more attention to electrolyte intake.
This means that for a 24-hour fast, most people don’t need to worry much about electrolyte supplementation beyond drinking water. The body’s regulatory systems are still handling the balance effectively at this stage.
What It Actually Feels Like
The physical experience at 24 hours varies, but common reports follow a pattern. The worst hunger typically hits between hours 8 and 16, coinciding with your normal meal times. By hour 24, many people describe feeling lighter, more mentally alert, and less fixated on food than they were at hour 12. Some people experience mild lightheadedness when standing up quickly, which relates to slight shifts in blood pressure and hydration rather than anything dangerous.
The mental clarity that many fasters report likely has several contributors: rising ketone levels (the brain runs efficiently on ketones), elevated norepinephrine keeping you alert, and the absence of post-meal blood sugar fluctuations that cause afternoon energy dips. The scientific evidence connecting fasting to a protein that supports brain health and cognition exists primarily in animal models and longer fasting protocols, so the sharpness you feel at 24 hours is more likely driven by hormonal shifts than structural brain changes.
Energy levels at this point are typically stable or even elevated, thanks to the metabolic rate holding firm and fat stores providing a steady fuel supply. Your body has about 40,000 or more calories stored as fat (even in lean individuals), so the fuel isn’t running low. Your body is simply learning to access it differently.