Why is my HRV low at night?

First: low compared to what?

HRV varies so much between people that a number on its own means little. What matters is how last night compares with your own recent nights [1]. One night below your normal is common and usually harmless. A drop that lasts several nights is more meaningful.

Second: low in which part of the night?

A nightly average can be low for two quite different reasons:

• Low at the start, normal later. HRV begins deep down and needs hours to climb. That usually points to load you took to bed — the pattern SAYA measures as the HRV-Incline.
• Low all night. The plateau itself is lower than usual. That points more towards longer-lasting load: accumulated training, ongoing stress, or your body fighting off an infection.

Common reasons for low overnight HRV

These factors are consistently associated with lower HRV during sleep:

• Alcohol. One of the clearest effects, and it grows with the amount [2]. More in HRV after alcohol.
• Late, intense exercise. Hard sessions that end close to bedtime are linked to lower HRV and higher heart rate in the night [3]. More in HRV after exercise.
• Evening stress. Work late into the evening, arguments, exciting screen time — a nervous system that is still activated takes longer to settle, and stress research links stress to more fragmented sleep [4].
• Late or heavy meals. Digestion is active work for the body.
• Accumulated training load. Several hard days in a row can lower your plateau even when single sessions felt fine.
• Illness. Infections often lower HRV and raise resting heart rate, sometimes before symptoms are obvious.
• Irregular sleep timing, travel and jet lag.

How to find your reason

The list above is generic. Which factor matters most differs from person to person — and that is exactly what your own data can show.

1. Log your evenings for a few weeks: dinner time, drinks, training, stress.
2. Compare your nights with and without each factor.
3. Change one thing at a time and see whether the pattern holds.

SAYA automates this: it compares your logged habits with your Garmin HRV and ranks the associations it finds. How that works is explained in personal sleep analytics.

Sources

1. Plews et al., 2013, Sports Medicine — HRV monitoring needs an individual baseline and longitudinal tracking. PMID 23852425
2. de Zambotti et al., 2020, Sleep — evening alcohol and nocturnal autonomic function, a dose–response laboratory study. PMC7819834
3. Leota et al., 2025, Nature Communications — dose-response relationship between evening exercise and sleep. s41467-025-58271-x
4. Grafe et al., 2023, Neurobiology of Stress — stress and sleep fragmentation (review of preclinical studies). PMC10709081

SAYA is a wellness product, not a medical device. Nothing here is a diagnosis or medical advice — if you suspect a sleep or heart condition, please talk to a professional.