During a 24-hour fast, your Garmin typically shows a gradually falling resting heart rate, a Body Battery that drains then recovers overnight, HRV that often rises as your body adapts, and a brief rise in stress around the 12-16 hour metabolic switch. Read together, these four metrics tell you where you are in your fast - which is exactly what FastFlow does automatically.
Most fasting apps show you a countdown timer and nothing else. But if you wear a Garmin, your wrist is already recording the physiological signature of your fast. You just need to know what to look for. Here's what each metric does across a 24-hour fast, and why it matters.
Resting heart rate: a slow, steady drop
As a fast progresses and digestion finishes, your resting heart rate usually drifts downward. Digestion is metabolically expensive, so once it's complete, your heart has less work to do. The lowest readings typically come overnight, in the Clearing stage (hours 4-12), when you're asleep and your body is quietly burning through glycogen.
A gently declining resting heart rate through a fast is a normal, healthy sign. A resting heart rate that climbs sharply, especially alongside dizziness, is a signal to pay attention and consider breaking the fast.
Body Battery: drain, then overnight recharge
Body Battery is Garmin's 0-100 estimate of your energy reserves, built largely from HRV, stress and activity. During a fast it behaves predictably: it drains through your waking hours and recharges overnight as you sleep through the Clearing stage. If you start a 24-hour fast in the evening, you'll often wake with a strong Body Battery already deep into the fast - the reason overnight fasting feels so easy.
By the 20-24 hour mark, Body Battery may sit lower than usual. That's expected on an extended fast; it's your body telling you to conserve.
HRV: the adaptation signal
Heart rate variability (HRV) - the variation in time between heartbeats - is the single most useful metric to watch during a fast. Higher HRV generally reflects a relaxed, well-recovered nervous system. Many fasters find their HRV holds steady or even rises during a well-run fast, particularly overnight.
The catch: HRV is deeply individual. A "good" number for you may be a "bad" number for someone else. That's why comparing your HRV to a population average is close to useless - and why FastFlow's on-device AI Coach compares each fast to your own baseline ("your HRV held 9ms above your usual") instead. We cover this in depth in HRV during fasting: what's normal, what's not.
Stress: watch the metabolic switch
Garmin's stress score is derived from HRV. During a fast, the most interesting reading comes around hours 12-16 - the Shifting stage, when your body switches from burning glucose to burning fat. Some people see a brief rise in stress here as the metabolic switch happens, often alongside a hunger wave. It usually settles once you're in ketosis in the Burning stage (16-24 hours).
Seeing that stress bump on your watch is oddly reassuring: it's visual proof the switch is underway, not a reason to worry.
Putting it together: a 24-hour fast, hour by hour
- 0-4h (Fed): slightly elevated heart rate from digestion; Body Battery high. More on the Fed stage.
- 4-12h (Clearing): resting heart rate at its lowest overnight; Body Battery recharging; HRV rising. More on Clearing.
- 12-16h (Shifting): possible stress bump and hunger wave as the metabolic switch happens. More on Shifting.
- 16-24h (Burning): low, steady heart rate; strong HRV; the calm, clear-headed ketosis window. More on Burning.
Why a wearable beats a timer
A timer tells you how long you've been fasting. Your Garmin tells you how your fast is actually going. That difference is the whole reason we built FastFlow: it reads these four metrics live and maps them onto the six metabolic stages, so you don't have to interpret raw numbers yourself. You just see your fast working.
Frequently asked questions
Does fasting lower your heart rate?
Resting heart rate often drifts downward during a fast, especially overnight, because digestion is complete and the heart has less work to do. A sharp rise in heart rate with dizziness is instead a signal to consider breaking the fast.
What happens to Body Battery when you fast?
Body Battery typically drains through your waking hours and recharges overnight during a fast. On an extended 24-hour fast it may sit lower than usual by the end, which is a normal sign to conserve energy.
Can my Garmin tell me when I'm in ketosis?
Not directly - Garmin doesn't measure ketones. But the pattern of a low steady heart rate and strong HRV in the 16-24 hour window lines up with the Burning stage, when the body is in ketosis. FastFlow maps these signals onto fasting stages for you.
Do I need a Garmin to use FastFlow?
No. FastFlow's timer, stages and journaling work without a wearable. A Garmin or Apple Watch adds the live biometrics - heart rate, HRV, Body Battery, stress - that drive the ring and power the AI Coach.
This article provides general wellness information and is not medical advice. Fasting isn't right for everyone. If you are pregnant, under 18, have a history of disordered eating, or manage a medical condition, consult a healthcare professional before fasting. This is a sensitive topic; if you are struggling with your relationship to food, consider reaching out to a qualified professional.