Human physiology is built around a predictable light–dark cycle. Hormones, body temperature, alertness, digestion, and recovery processes all follow a roughly 24-hour rhythm known as the circadian rhythm.
Shift work — especially night shifts or rotating schedules — disrupts this rhythm. Even when someone is disciplined with training, nutrition, and sleep, the misalignment between biology and schedule can create a hidden recovery load that shows up clearly in HRV and resting heart rate.
Shift work isn’t just tiring. It is a chronic physiological stressor.
Your Nervous System Runs on a Clock
The autonomic nervous system follows circadian patterns.
In a typical rhythm:
• Cortisol rises in the morning to help you wake up
• Body temperature peaks during the day
• Melatonin rises at night to promote sleep
• Parasympathetic activity increases overnight for recovery
When you’re awake, alert, and working during the biological night, and trying to sleep during the biological day, these patterns become misaligned.
The nervous system receives mixed signals about when to be active and when to recover.
Sleep During the Day Is Not the Same
Even if total sleep time looks decent, daytime sleep is often:
• Lighter
• More fragmented
• Shorter in deep sleep phases
Light exposure, noise, and internal circadian signals make it harder to achieve the same quality of recovery that occurs during nighttime sleep.
This can lead to:
• Lower overnight HRV
• Higher resting heart rate
• Feeling less restored despite “enough hours” in bed
Hormonal Rhythms Become Blunted
Shift work can disrupt the natural rhythm of key hormones involved in recovery, including:
• Cortisol
• Melatonin
• Growth hormone
Cortisol may stay elevated at times when it would normally drop, while melatonin release may be suppressed by light exposure at night.
This hormonal mismatch increases overall stress load and reduces the body’s ability to fully downshift into recovery mode.
HRV Often Reflects This Hidden Stress
Because HRV reflects autonomic balance, shift workers often see:
• Lower average HRV
• More day-to-day variability
• Slower recovery after hard sessions
This can feel confusing, especially when training load hasn’t changed. The missing piece is circadian stress, not just physical stress.
Training Recovery Is Harder to Predict
When sleep timing shifts from day to day (common with rotating schedules), recovery becomes less consistent.
Some days the nervous system may be more suppressed simply due to poor circadian alignment, not because of excessive training.
That’s why HRV trends can be especially useful for shift workers — they help reveal how much total load the system is carrying.
What Shift Workers Can Do
While shift work itself may not be optional, some strategies can reduce its impact:
• Keep sleep timing as consistent as possible, even on days off
• Use blackout curtains and cool temperatures to improve daytime sleep
• Get bright light exposure soon after waking (even if that’s in the afternoon)
• Limit bright light exposure before planned sleep
• Emphasize low-intensity aerobic work on lower-HRV days
• Avoid stacking multiple very intense sessions during poor-sleep stretches
These steps don’t eliminate circadian disruption, but they can reduce the recovery cost.
The Big Takeaway
Shift work challenges the body’s natural circadian rhythm, making recovery less efficient even when effort and discipline are high.
Lower HRV and higher resting heart rate in shift workers often reflect circadian stress rather than just training stress. Managing light exposure, sleep consistency, and training intensity can help the nervous system cope with this ongoing load.