Why is my HRV different in summer and winter?

Two clocks move your HRV. A 24-hour body clock body clock Circadian rhythm The body's 24-hour clock. Heart rate is lower and vagal HRV higher at night than by day, so readings are only comparable when they are taken at the same time of day. More about Circadian rhythm makes vagal HRV higher at night and lower by day, and a slower seasonal drift shifts it a little over the year. The seasonal part is small: in one study of 120 healthy men, RMSSD 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 differed by about 3 ms between summer and winter. Most of what you notice comes from what changes with the season.

The daily clock

Your body runs on a clock of about 24 hours that times sleep, temperature, hormones and heart rate. HF power HF power HF power The variance of the heartbeat intervals that falls in the 0.15 to 0.40 Hz band, in ms²: the range of normal breathing. It mainly reflects vagal activity when breathing is 9 to 24 breaths a minute. More about HF power , the breathing-linked part of HRV, tends to rise at night and fall during the day (Shaffer and Ginsberg, 2017). The clock keeps time even when sleep is moved. Boudreau and colleagues (2013) kept 13 young adults on short sleep-wake cycles in time isolation for 72 hours: heart rate and HF power still followed a circadian rhythm, and vagal activity vagal activity Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone during deep sleep peaked at about 02:00.

That is why the time of your reading matters more than anything else on this page. A morning reading soon after waking and an overnight average each work, as long as you stick to one.

The model below builds a simulated day with a 24-hour swing in heart rate. Over a 5-minute window SDNN 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 is about 30 ms; stretch the window to 24 hours and the day-night swing pushes it close to three times that, while RMSSD barely moves.

How long should you record?

A simulated day, not a person

Value over the window, ms RMSSD SDNN

HF LF 5 min α2 24 h

Window length 5 min 10 s 5 min 1 h 24 h

RMSSD

37.2 ms

SDNN

29.7 ms

Beats in the window

327

Over 5 min, RMSSD is 37.2 ms and SDNN 29.7 ms. This is the short-term standard.

The slider works when JavaScript is on.

A chart of RMSSD and SDNN against recording length on a logarithmic axis from 10 seconds to 24 hours. The RMSSD line stays close to 37 ms at every length. The SDNN line sits near 30 ms up to a few minutes and climbs to about 84 ms at 24 hours. Dashed marks show the minimum lengths: 1 minute for the HF band, 2 minutes for LF, 5 minutes for the short-term standard and the VLF band, 20 minutes for DFA alpha 2, and 24 hours for ULF and long-term SDNN. A slider picks the window length, and readouts give RMSSD, SDNN and the number of beats in the window.

The seasons

Kristal-Boneh and colleagues (2000) recorded 24 hours of heartbeats in 120 healthy men, once in summer and once in winter. The average gap between beats was the same in both seasons, yet every HRV measure differed: RMSSD by about 3 ms and the spread of 5-minute averages by about 14 ms. Those are small changes next to normal day-to-day noise. Even this study is unclear on the direction: its summary reports HRV lower in summer in one sentence and lowest in winter in its conclusion.

Not every source agrees on the direction. Marco Altini, the scientist behind the HRV4Training app, writes that HRV “tends to be higher in summer months”, and notes that better weather may also mean more training. Seasons shift HRV a little, the direction differs between people and places, and there is no fixed seasonal rule to correct for.

What changes with the season

The things that move with the calendar are better explanations for a seasonal change in your own data:

• Heat. A sudden heat wave often lowers HRV for a few days; repeated heat exposure leads to adaptation (Altini, 2025).
• Training. Better weather may mean more training (Altini, 2025), and many sports follow a racing season.
• Light and sleep. Shorter nights in summer and darker mornings in winter shift sleep and waking times.
• Illness. Colds and flu are more common in winter, and illness lowers HRV for days.

Go deeper Why the slow rhythms need a long recording

The day-night swing is part of the ultra-low-frequency band of HRV, below 0.003 Hz. It can only be measured from a recording of 24 hours or longer (Shaffer and Ginsberg, 2017). A short morning reading cannot see it at all; it only sees which point of the clock you measured at.

How to read your trend across seasons

1. Keep the time and position of your reading fixed all year.
2. Judge each week against your recent baseline baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline , not against last summer.
3. When your average shifts with the season, look for the change in your life that came with it: training, heat, sleep or a cold.
4. If a drop comes with feeling unwell for more than a few days, or with symptoms such as chest pain or breathlessness, talk to a doctor; HRV cannot tell you the cause.

For why readings bounce from day to day, see why does my HRV change so much?