Resting heart rate
What it is: Your resting heart rate (RHR) is a measure of how many times your heart beats per minute when you’re awake, relaxed, and resting. Normally, it sits somewhere between 60 and 100, though if you’re very fit it can be lower. On the other hand, a higher RHR can indicate that you’re stressed, dehydrated, or unwell.
How it may change when using a GLP-1: GLP-1 treatment may cause a small increase in RHR, typically by 2-4 beats per minute, often appearing around eight weeks into treatment. This change happens because GLP-1s can affect how your nervous system works, specifically the balance between two parts that control your heart rate:
- They stimulate your sympathetic nervous system (responsible for the ‘fight-or-flight’ response), which tells your heart to beat faster.
- They inhibit your parasympathetic nervous system (also called the ‘rest-and-digest’ system), which decreases your heart rate.
The result of this is that your heart may beat a little faster at rest, and you see a slight increase in your RHR.
What the tracker may show: Depending on the type of tracker you use, your app might indicate lower readiness or recovery scores, that your body needs more time to recover, or that your nervous system is spending more time in ‘stress mode’ rather than ‘recovery mode’.
What to do: It may help to add a little more movement to your daily routine, as research has shown that even modestly higher levels of physical activity (around 10 minutes or more per day) may help to temper increases in RHR linked to GLP-1 medications.
But if your tracker shows consistent increases in RHR of over four beats per minute, seek medical advice, as a higher RHR is a risk factor for life-threatening heart problems.
Active heart rate
What it is: Your active heart rate is how fast your heart beats per minute while you’re moving or exercising. It basically shows how hard your heart is working to supply oxygen to your muscles when you’re physically active.
How it may change when using a GLP-1: There isn’t much data on how GLP-1s can affect active heart rate, but small variations between workouts are typically normal.
What the tracker may tell you: Higher average heart rate during workouts, especially early on while your body adapts.
What to do: Small fluctuations are normal and nothing to worry about, but if your active heart rate feels really high compared with the level of effort you put in, or if you experience dizziness, chest pain, or shortness of breath when you exercise, speak with your clinician. They might want to monitor you more closely, adjust your dose, or stop the treatment altogether.
Sleep quality
What it is: Sleep quality refers to how well you sleep through the night, with trackers estimating this by monitoring your movements, heart rate, and sometimes breathing trends to determine how much time you spend in light, deep, and REM sleep.
These are, of course, just estimates, so trackers may not be as precise as a full sleep study using a device that monitors brain waves, heart rate, and eye movements.
How it may change when using a GLP-1: Some people report sleep disturbances or insomnia, especially when starting therapy with GLP-1s like Wegovy and Mounjaro. Research hasn’t uncovered why this happens yet, but one possible explanation is that side effects like nausea, bloating, and acid reflux can cause discomfort at night, which can disrupt sleep.
In the long run, though, GLP-1s can also improve sleep quality by reducing problems like obstructive sleep apnoea.
What the tracker may tell you: Your sleep score is lower than usual, and your sleep quality is low (more awake time, less deep sleep). It might also pick up changes in sleep duration or heart rate during sleep.
What to do: If you experience sleep disruptions, try to improve sleep hygiene (stick to a consistent bedtime, sleep in a cool dark room, limit screens 30 minutes to an hour before bed).
And if you’re struggling with persistent insomnia, loud snoring, gasping during the night, or excessive daytime fatigue, it’s a good idea to talk to your doctor, especially if these issues don’t resolve as your body adjusts to the medication.
Calorie burn rate
What it is: Calorie burn rate is how many calories your body uses over a certain period of time (usually 24 hours). Fitness trackers typically use your activity level, heart rate, age, weight, and sometimes other factors to estimate it.
How it may change when using a GLP-1: There’s no evidence at the moment to suggest that GLP-1s affect metabolic rate beyond what you’d expect with weight loss itself. If your tracker shows fewer calories burned, it’s usually because, as you lose weight, your body becomes smaller and naturally burns fewer calories, not because of a direct effect of the GLP-1.
What the tracker may tell you: You’re burning fewer calories than usual.
What to do: If your tracker shows dramatic drops in metabolic rate or fatigue, consider reviewing your diet and activity levels with a clinician or dietitian, especially if you’re losing weight faster than recommended (the NHS recommends a rate of 0.5 to 1 kg or 1-2 lbs a week).
Body temperature
What it is: Body temperature is a measure of how warm your body is. It’s usually around 36°C to 37°C, and your body works pretty hard to keep it stable to allow your organs and systems to function properly.
Some wearables estimate skin or peripheral temperature (such as wrist or finger temperature), which can fluctuate with:
- Sleep cycles
- Fever or illness, when it generally goes up
- Hormonal shifts, like after ovulation (when it goes up) or during the follicular phase of the menstrual cycle (when it may be a bit lower than usual)
- Changes in the environment, like ambient temperature
These readings aren’t always the same as basal body temperature (BBT), but studies show they can be a pretty reliable way to estimate it. They can, however, be affected by how and where the sensor sits on your body.
How it may change when using a GLP-1: There’s no solid research on the effects of GLP-1s on body temperature, but anecdotal reports suggest that GLP-1 therapy may cause them to be more sensitive to cold.
What the tracker may tell you: It may display body temperature changes (+/-) from your personal baseline.
What to do: If you’re feeling cold, keep yourself warm by layering your clothes, keeping up your protein intake to protect muscle, doing regular strength exercises, and enjoying warm drinks in a cozy indoor space.
If, on top of cold sensitivity, you notice severe tiredness, hair loss, brain fog, or other unusual symptoms, check in with a clinician. These symptoms could suggest thyroid problems or other medical conditions.
And if your body temperature is 38°C or above while resting, your body might be fighting an infection. In this case, it’s best to drink plenty of fluids, take paracetamol or ibuprofen and seek medical help if the fever persists.
