Often, although the studies do not all agree. A 2023 review gathered 12 studies of 24-hour shift work, with 8 to 60 people each: of the nine that reported the LF/HF ratio LF/HF ratio LF power divided by HF power, with no unit. It was proposed as a gauge of balance between the sympathetic and parasympathetic systems, but research shows it cannot measure that reliably. More about LF/HF ratio , three found it higher after the shift, and of the five that reported high-frequency 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 , two found it lower. The authors called the findings inconsistent but pointing toward a shift away from parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system dominance (Jelmini and colleagues, 2023). The clearer effect is on the daily rhythm of heart rate variability (HRV), which does not adapt to night work after one to three nights in a row (Jensen and colleagues, 2016).
Why night work moves HRV
HRV follows the 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 . It rises at night, when the vagal brake dominates, and falls in the morning hours. A night shift asks you to be alert through the hours your body expects to rest, and to sleep through the hours it expects to be up. Sleep in the day is shorter and lighter, 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). A 2024 review of the factors that shape HRV lists shift work among the external factors linked with lower values (Sammito and colleagues, 2024).
What the reviews found
Long shifts. The 2023 review searched the literature from 1964 to 2023 and found only 12 studies that met its criteria, in firefighters, ambulance crews, doctors and other 24-hour workers. The pooled effect on the LF/HF ratio was a moderate rise (Hedges’ g = 0.82), but its confidence interval crossed zero, so it was not statistically significant, and the authors put the inconsistency down to differences in recording lengths, equipment and job demands. Five studies were rated high quality, five moderate and two low.
Consecutive nights. The 2016 review looked at field studies that tracked daily rhythms in cortisol, melatonin and HRV across runs of night shifts. Most were small, with fewer than 30 people, and most tested only two nights in a row. Across them, the rhythms had not adapted after one to three consecutive nights. A small number of studies found full adaptation after seven nights for cortisol and melatonin. For HRV, the review had four studies and could not say how many nights adaptation takes.
The practical reading: on a typical rota of two to four nights, your body clock stays on day time, so your HRV during a night shift sits where your waking daytime HRV would, and your day-sleep HRV does not reach the night-time peak it would on a normal night.
How to read your own data
- Build two baselines. Keep readings from shift blocks and from days off apart, and compare each with its own usual range. The lesson on building your HRV baseline shows how a range is built; the same maths works on either set.
- Judge the shift block by its trend. A steady fall across a run of nights that recovers on days off is the expected pattern. A block that stays low into the days off, or a trend that drifts down across months, is worth a closer look at sleep.
- Protect day sleep. A dark, cool, quiet room and a fixed sleep window after nights make day sleep longer, and sleep length is the lever with the clearest evidence.
- Watch the companions. Night work often brings late caffeine, heavy meals at odd hours and less exercise. Each has its own page: caffeine, sleep, stress.
Travel across time zones moves the body clock in a similar way; see does altitude, flying or jet lag change HRV?
When to talk to a doctor
Talk to a doctor if you cannot sleep in the day despite a good routine, if you fall asleep at the wheel or at work, or if a low mood sets in with shift work. Get checked for chest pain, palpitations or breathlessness whatever your HRV shows. HRV cannot diagnose anything, and this page is not medical advice.