A little, and not clearly for the worse. The best study so far followed 58 adults with treated high blood pressure through the last two weeks of Ramadan and again a month later, with a 24-hour heart monitor each time. In the afternoon, SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN was 59 ms while fasting against 44 ms when not, and an HRV-based stress index Baevsky stress index A number built from the histogram of heartbeat intervals, with no unit. It rises as the rhythm gets steadier and less variable, which Baevsky read as strain on the body's regulation. More about Baevsky stress index was lower (Hammoud and colleagues, 2021). That is one small, observational study, and for popular patterns such as 16:8 or alternate-day fasting there is no heart rate variability (HRV) trial at all.
What the Ramadan study found
The 58 participants (30 men, 28 women, average age 54) kept taking their blood pressure medicines through the month. The monitors recorded whole days, which let the researchers compare the same hours fasting and not fasting:
| Time of day | During Ramadan | A month later |
|---|---|---|
| Afternoon SDNN | 58.6 ms | 44.4 ms |
| Afternoon stress index | 279 | 350 |
| Evening mean beat interval | 707 ms (faster heart) | 755 ms |
The afternoon, deep into the fast, looked calmer than usual. The evening, after the fast was broken with a large meal, had the fastest heart rate and the lowest SDNN of the whole day. Blood pressure, body composition and cholesterol did not change, while blood glucose rose a little.
The study has real limits. It had no reading from before Ramadan, so the “after” month stands in for normal. It was small, everyone had high blood pressure, and the medicines varied. The authors read the afternoon rise as less cardiac stress during the fast, which is a reasonable reading of one study, not a settled fact.
What fasting might be doing
Several things move at once during a fast, and they do not all point the same way:
- Fewer meals during the day. Digestion shifts blood to the gut and raises heart rate for a while. Skipping that may leave the afternoon calmer.
- A big meal late. In a Turkish crossover study, 16 young men ate a large sweet or a large protein-and-fat meal at 10 p.m.; the heavy meal disturbed their sleep, although a 5-minute HRV recording did not differ (Uçar and colleagues, 2021). Overnight recordings may tell a different story.
- Lost sleep. Late meals and early starts shorten sleep, and sleep loss lowers RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD across 11 trials (Zhang and colleagues, 2025). This is the most likely reason for low night readings during a fasting month.
- Less alcohol and fewer snacks. Many people who start a fasting routine also drop the habits that lower overnight HRV most, so a rise may be about what they stopped, not the fasting itself.
How to read your own data
- Tag fasting days and fasting weeks. Then compare the 7-day average with your usual range. The lesson on building your HRV baseline shows how.
- Look at the clock. If your device shows HRV through the night, a low first hour after a big late meal with a normal rest of the night fits the Ramadan pattern.
- Check sleep length. If fasting days come with shorter sleep, the sleep is the more likely cause of any drop. Read HRV and sleep.
- Expect dehydration to count. Long dry fasts in hot weather can lower HRV through fluid loss. See does dehydration lower HRV?
For weight loss itself, which has better evidence, read does losing weight raise HRV?
When to talk to a doctor
Ask a doctor before fasting if you have diabetes, a heart condition, kidney disease, a history of an eating disorder, or if you are pregnant or breastfeeding, and about the timing of any medicines. Stop and get help if you feel faint, confused, have chest pain or a racing heartbeat. HRV cannot tell you whether a fast is safe for you, and this page is not medical advice.