ULF power

ULF power is the amount of heartbeat-interval variance at frequencies below 0.003 Hz, in ms²: everything that cycles more slowly than once every 5 minutes or so.

Why this measure exists

A spectrum of a whole day holds rhythms no 5-minute recording can see: the rise and fall of heart rate between waking and sleeping, the hours of activity and rest, meals and hormones. The 1996 Task Force’s 24-hour table gives this slowest band its own name. In Bigger and colleagues’ 1992 study of 715 people after a heart attack, ULF was the single strongest spectral predictor of death over four years, ahead of VLF VLF VLF power The variance of the heartbeat intervals in the 0.003 to 0.04 Hz band, in ms²: rhythms repeating every 25 seconds to 5 minutes. It needs at least a 5-minute recording and is best read over 24 hours. More about VLF power , LF LF LF power The variance of the heartbeat intervals that falls in the 0.04 to 0.15 Hz band, in ms²: rhythms repeating every 7 to 25 seconds. Once read as sympathetic activity, which is now disputed. More about LF power and HF HF 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 . Shaffer and Ginsberg name the circadian rhythm circadian rhythm 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 as the likely main driver, with core temperature, metabolism and the renin–angiotensin system possibly contributing; whether the two autonomic branches add to it is disputed.

The formula

$$\mathrm{ULF} = \int_{0}^{0.003} P(f)\,\mathrm{d}f$$

Add up the power spectral density $P(f)$, in ms² per Hz, over every frequency $f$ below 0.003 Hz. The fastest wave the band holds takes about 5.6 minutes; to resolve anything inside the band a recording must be many times longer, which is why the Task Force reserves ULF for 24-hour recordings.

Worked example

Take the rhythm apart

Five minutes of the teaching recording

Power at each frequency, Hz Breathing

VLFLFHF

0 0.1 0.2 0.3 0.4 0.5

ULF

needs 24 h

VLF

3 ms²

LF

120 ms²

HF

635 ms²

Total

758 ms²

LF nu

15.9

HF nu

84.1

LF/HF

0.19

Peak

0.250 Hz, HF

Rested, 15 breaths a minute (0.250 Hz), 5 minutes, Welch's method after detrending. The breathing peak sits at 0.250 Hz in the HF band, with the ~0.1 Hz blood-pressure wave as a smaller LF peak. LF/HF is 0.19.

Rested Stressed Drifting Ectopic beats

Breathing rate 15 breaths a minute Recording length 5 minutes

Welch (FFT) Lomb–Scargle

Detrend first (smoothness priors, λ = 500)

The controls work when JavaScript is on.

A line chart of power against frequency from 0 to 0.5 Hz with the VLF (0.003–0.04 Hz), LF (0.04–0.15 Hz) and HF (0.15–0.4 Hz) bands shaded and a marker at the breathing frequency. Readouts give ULF, VLF, LF, HF and total power in ms squared, LF and HF in normalised units, the LF/HF ratio and the peak frequency, followed by a sentence explaining what changed. Controls: a breathing-rate slider from 5 to 20 breaths a minute, a recording-length slider, a detrending checkbox, Welch or Lomb–Scargle buttons, and four preset buttons for a rested, stressed, drifting or ectopic recording.

The explorer works on 5 minutes of the teaching recording, so its ULF readout says "needs 24 h": there is no frequency bin below 0.003 Hz in a 5-minute spectrum, and anything slower than the recording is removed as a trend before the transform. The longest recording the explorer offers is still a hundred times too short. Over 24 hours, SDANN SDANN SDANN The standard deviation of the mean NN interval of each 5-minute segment of a 24-hour recording, in milliseconds. It measures the slow changes over a day and ignores everything faster. More about SDANN , the standard deviation of the 5-minute averages, tracks the same slow changes and is far easier to compute.

What moves it

The shape of your day: how much heart rate differs between sleep and waking, between a desk and a walk. Shift work, a sleepless night or a day in bed change it; the beat-to-beat swings of breathing do not.

When it misleads

Whenever a program prints a ULF value for a short recording: it is the trend, not a rhythm. And like every 24-hour measure it depends on what you did that day, so a single day cannot be compared with another person’s, or with your own day spent differently.

Where you will see it

The Task Force’s 24-hour table lists ULF. Kubios HRV’s short-term analysis stops at VLF; Holter software and research papers on 24-hour recordings report it, often as its log. Wearables do not show it.