Circadian rhythm
Also called circadian, body clock, 24-hour rhythm, day-night 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.
Your body runs on a clock of about 24 hours that times sleep, body temperature, hormones and heart rate. The clock keeps going even when sleep is moved. Boudreau and colleagues (2013) showed this by putting 13 young adults on short sleep-wake cycles in time isolation for 72 hours: heart rate and 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 still followed a circadian rhythm, awake and in every non-REM sleep stage, and 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.
Shaffer and Ginsberg (2017) summarise the daily pattern: HF power tends to rise at night and fall during the day. Over 24 hours this slow swing is part of the ultra-low-frequency band, which can only be measured with a recording of a day or longer (Shaffer, McCraty and Zerr, 2014).
Why it matters for HRV.
- Same time, every time. A reading at 7:00 and one at 18:00 sit at different points of the clock. Morning readings soon after waking and overnight averages each work, as long as you stick to one.
- Long recordings mix the clock in. 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 over 24 hours includes the day-night swing, which is why it is much larger than a 5-minute SDNN. The model below builds a simulated day with a circadian swing in heart rate; drag the window from 5 minutes to 24 hours and watch SDNN grow.
- Sleep at a new time does not move the clock at once. In the study above the rhythm held while sleep came at all hours, so a reading after a late night or a flight across time zones is taken at a different point of the clock.
How long should you record?
A simulated day, not a person
- 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.
Why is my HRV different in summer and winter? covers the daily rhythm and the slower, seasonal one.