How to do a 24 hour fast

A 24 hour fast means going one full day without eating, for example from dinner at 7 pm to dinner at 7 pm the next day. Water is the baseline. If you have a medical condition or take medication, check with your doctor first.

Who should not do this

When Harvard Health says time-restricted eating appears safe for most people, it excludes people with diabetes, people with eating disorders, pregnant or breastfeeding people, and people who need to take medication with food. Fasting Timer's own disclaimer also asks anyone under 18 to talk to their doctor before fasting.

If any of that is you, or you aren't sure, talk to your doctor before you try it.

Pick your 24 hours

The simplest way is to choose a meal and wait until the same meal the next day.

What you can drink

Plain water is the baseline. Two registered fasting studies went further. In the University of Florida's Time to Eat pilot, participants could have calorie-free beverages, water, tea, black coffee or sugar-free gum during the fasting period. In the University of Bath's intermittent fasting study, fasting participants only consumed water and black tea or coffee, with no milk or sugar. Those are study protocols, not health advice. We cover the coffee question in can you drink coffee while fasting. The Florida record (NCT03590847) is a 4-week pilot with 10 adults aged 65 and older who have overweight and limited mobility. The Bath record (NCT02498002) enrolled adults aged 18 to 65, lean and with overweight.

If a doctor asked you to fast for a blood test, it's a different fast. MedlinePlus says to have nothing but plain water, and that coffee can affect your results.

What the research says about hours 12 to 36

A review in Obesity (Anton et al., 2018) describes a "metabolic switch" in which the body moves from using glucose to using fat and ketones as fuel. The authors say it typically happens between 12 and 36 hours after you stop eating. The exact point depends on the glycogen in your liver when you start and on how much energy you burn during the fast. They also note that when insulin resistance rises, the switch takes longer. The review also says people in some trials reported high levels of hunger and potential discomfort on fasting days, and that safety remains a potential concern.

So at hour 24 you might be past that switch or approaching it, and the clock alone can't tell you which.

Hunger and the first meal

A small randomized crossover study in PLoS One (Cameron et al., 2014) compared a fed day with a fast in 15 adults (mean age about 29; eligibility was ages 18 to 40, non-diabetic). The fast ran from 12:00 on one day to 11:00 the next, slightly less than 24 hours. Over the test morning, hunger ratings, summarized as area under the curve, were 69% higher in the fasted condition and fullness was 47% lower. Both conditions then had the same standard lunch, two slices of cheese pizza. At a dessert buffet afterward, participants ate more in the fasted condition, about 855 kcal versus 491 kcal. Body weight didn't change between conditions.

It was a small lab study, so it shows what can happen, not what will happen to you. None of the sources used here gives a protocol for ending a fast. Writing about time-restricted eating, Harvard Health warns that gorging during the eating window keeps it from working as a weight-loss strategy.

If you're fasting for a blood test

MedlinePlus says you usually fast for 8 to 12 hours before a test, with nothing but plain water, and your provider confirms the exact time. Follow their instructions, not this guide.

Doing a 24 hour fast with Fasting Timer

Fasting Timer runs on iPhone. The timer and the daily plans, from 12:12 to OMAD, are free. The 24 hour plan is one of the Extended fasts, which are part of Premium.

1. Open the Timer tab and tap the plan chip (it shows the current plan, such as "16:8 · Most popular").
2. In Choose a plan, find Extended fasts and tap 24h fast ("A full day"). Without Premium, it opens the upgrade screen instead.
3. Tap Start fasting. The ring fills as time passes, and the Started and Goal boxes show when you began and when you'll finish.
4. Started late? Tap the Started box to open Edit start time and tap Save.
5. Notifications tell you when the fast is complete and when there's one hour to go.
6. At the end, tap Complete fast. If you stop early, End fast asks "End your fast?" and offers End and save or Discard this fast.

The Current stage card on the Timer tab names a stage for each point of the fast; at 24 hours it reads "Deep autophagy". Treat those stages as rough. The app says its timings are approximate and general information, and the sources cited here don't support those labels, only the 12 to 36 hour range above.

The free OMAD plan is 23 hours of fasting and one hour of eating, if you want something close to a full day without Premium. The free version keeps your last 7 days and your current streak. There are also Home Screen and Lock Screen widgets, and a Water today card on the Timer tab to log water. Fasting Timer doesn't count calories, give meal plans or sync across devices; your data stays on your iPhone.

Fasting stages are general information, not medical advice. Talk to your doctor before fasting, especially if you are pregnant, have diabetes or another medical condition.