Does the flu shot make you tired? What happens to your heart rate, HRV and sleep

Short answer

Yes, the flu shot can make you tired, but usually only a little and only for a day or two. In a study of Israeli adults who reported their reactions for 72 hours, weakness was the most common whole-body reaction after a flu shot, at 7.3%, and two in three people (66.9%) had no whole-body reaction at all (1, 2). The CDC says flu shot side effects are generally mild and go away on their own within a few days (3).

Your heart rate and HRV usually barely move. In the same study, participants wore Garmin smartwatches, and the flu shot caused no statistically significant change in heart rate or in an HRV-based stress score, while a COVID-19 booster raised heart rate by about 4.5 beats per minute at its peak (2).

If you feel wiped out after a shot that "shouldn't" do much, you're not imagining it, and it's not weakness. Bodies react differently, and a tired day can be part of a normal immune response.

Note: this article is for general education and is not medical advice. Trouble breathing, wheezing, swelling of the face or lips, hives, a racing heart or dizziness within minutes to hours of a shot can be a severe allergic reaction: call 911. This guide is published by Welltory, which makes one of the apps mentioned.

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Why the flu shot can make you tired

A flu shot shows your immune system a harmless piece of the virus, or an inactivated virus, so it can practice. The CDC states plainly that flu vaccines cannot cause flu (3). But practice still takes effort.

In the first day or so, immune cells at the injection site send out signaling molecules. Some of them reach the brain and produce what doctors call "sickness behavior": you feel slower, want to rest, and may sleep more. It's the same mechanism that makes you tired with a real infection, at a much smaller scale.

Other common reasons you feel tired after a flu shot:

• A sore arm that wakes you when you roll onto it.

• A low fever or chills, which disturb sleep.

• Headache or muscle aches.

• The day itself. Many people get vaccinated after work, on a rushed lunch break or at the end of a long week.

• A cold you were already catching. Flu shots are given in fall, when colds, RSV and COVID-19 also circulate. Adults in the US catch two to three colds a year on average (4), so some of them will line up with a vaccine date by chance.

How common is tiredness after a flu shot?

The best numbers come from studies that asked people every day, not only those who called a doctor.

Two things stand out. First, most people felt nothing beyond the arm. Second, people who reacted more strongly to the flu shot also tended to react more strongly to a COVID-19 booster (1). If you're "a reactor", that likely reflects how your own immune system responds.

How long does tiredness last after a flu shot?

For most people, a day or two. The CDC describes side effects that go away on their own within a few days (3). The study above tracked reactions over 72 hours for this reason.

A useful rule of thumb:

• Day 0–1: sore arm, possibly feeling heavy or sleepy in the evening.

• Day 1–2: the most likely time for tiredness, headache or a low fever.

• Day 3 onward: you should feel back to normal.

If tiredness gets worse after day 2, or new symptoms appear (sore throat, cough, runny nose, high fever), think about other causes. Tiredness that lasts a week isn't a typical flu shot reaction, and it's worth a call to your doctor. For the full list of what's common and how long it lasts, see our guide to vaccine side effects.

What happens to your heart rate after a flu shot

On average, almost nothing measurable. This is the most reassuring finding for anyone who tracks their heart.

In the Israeli smartwatch study, participants wore a Garmin Vivosmart 4 before and after vaccination (2):

• After the flu shot: no statistically significant change in heart rate.

• After the COVID-19 booster: heart rate rose over the first three days, peaking 22 hours after the shot at 4.48 beats per minute above baseline, and returned to baseline by day 6.

• Same person, both vaccines: on day 2 the COVID-19 rise was 1.5 beats per minute higher than after the flu shot, and the difference was gone by day 3.

That doesn't mean your heart rate can't change. Averages hide individuals. If you develop a fever, your heart rate will rise with it: in a classic study, heart rate went up by about 8.5 beats per minute for every 1 °C (1.8 °F) of fever (5). A poor night with a sore arm can add a few beats the next morning too.

What about palpitations? A brief sense of a faster heartbeat can come with fever, anxiety around needles, or caffeine on vaccination day. But a racing heart together with trouble breathing, swelling, hives or dizziness within minutes to hours of the shot is a possible severe allergic reaction, and the CDC advises calling 911 (3). Palpitations that keep coming back or an irregular pulse days later deserve a doctor's visit, whatever the cause. Our article on high resting heart rate causes covers other reasons it climbs.

What happens to HRV after a flu shot

HRV (heart rate variability) reflects how your nervous system steers your heart. When your body deals with inflammation, HRV often dips for a while.

For the flu shot, the change seems small. Garmin's stress score is built from HRV, and in the Israeli study it showed no statistically significant change after flu vaccination (2). After a COVID-19 booster, the same stress score rose, peaked 22 hours after the shot and returned to baseline within six days (2).

How to read your own HRV after a flu shot:

1. Compare with your own baseline. Look at your average for the 1–2 weeks before the shot, measured the same way.

2. Expect noise. HRV naturally moves from day to day. Our guide on why HRV fluctuates day to day shows how much.

3. Look at the next 1–3 mornings. A dip that lasts a day or two and comes back is consistent with a normal immune response.

4. Watch the trend, not the score. If HRV stays low for a week or more and you feel worse, look for another cause. Our article on low HRV causes lists the usual suspects.

What happens to sleep after a flu shot

Sleep is where most people notice a flu shot, mainly because of a sore arm or a restless, warm night.

Sleep after the shot. You may want to sleep earlier or longer. That's fine. Side-sleepers can try the opposite side, or a pillow that keeps the arm from bearing weight.

Sleep before the shot matters too. In a classic 2002 study, people whose sleep was restricted around a flu vaccination had about half the antibody levels of well-rested people 10 days later (6). A 2023 meta-analysis of seven studies found that sleeping fewer than six hours a night around vaccination was linked to a weaker antibody response; the authors estimated the effect as roughly equal to two months of antibody waning (6). The link was clear in men and weaker, and not statistically significant, in women, and it showed up only when sleep was measured objectively, with a wearable or in a sleep lab (6).

That's a reason to book your shot for a week when you can sleep normally, not a reason to skip the shot after a bad night.

How to plan your flu shot around your energy

You can't control your immune response, but you can make the next 48 hours easier.

1. Pick a lighter day. Avoid the day before a big deadline, a long drive or a race. Morning or early afternoon gives you an evening to rest if you need it.

2. Sleep normally the nights before. Aim for your usual sleep, not a short night.

3. Eat and drink normally. Fainting during or after a shot is more likely if you're anxious, hungry or dehydrated.

4. Move the arm afterward. Gentle movement often eases soreness.

5. Keep training light for a day. A walk is fine. If you feel feverish or achy, skip the hard session; our guide to working out while sick explains the "above the neck" rule.

6. Note the date. Write down the day and time of the shot, so you can match it with how you feel and what your data show.

7. Plan for others. If you're getting several vaccines in one visit, expect a stronger response than after one shot.

When to get the shot, which type fits your age, and whether it's too late this season are covered in our guide on when to get a flu shot.

Flu shot tiredness or something else?

Tiredness after a flu shot has a short, predictable shape. Other causes look different.

A cold or flu picked up around the same time is the most common "something else". Fall is respiratory virus season.

Living with flu shot side effects: the questions people ask next

Can a flu shot make you tired a week later?

It's not a typical reaction. Side effects usually go away within a few days (3). Tiredness that starts or worsens a week later more likely has another cause, such as a cold, poor sleep or stress.

Should I work out after a flu shot?

Light movement is fine and may ease a sore arm. If you feel feverish, achy or unusually tired, skip hard training for a day or two.

Why do I feel worse after a flu shot than my friends?

People differ. In one study, those who reacted more to a flu shot also reacted more to a COVID-19 booster. A stronger reaction doesn't mean something is wrong with you or with the vaccine.

My resting heart rate went up after my flu shot. Is that normal?

A small, short rise can happen, especially with fever or a poor night. On average, smartwatch data show no significant change after a flu shot. If it stays high for days or comes with dizziness or breathlessness, call your doctor.

Is it safe to get a flu shot if my HRV is low that day?

A low HRV reading alone isn't a reason to skip a vaccine. If you have a fever or feel unwell, ask the pharmacist or doctor whether to wait.

Before you blame yourself

If a flu shot leaves you flat for a day, it's easy to think "I'm out of shape" or "my body can't handle anything". Before you blame yourself, look at your data. Did you sleep badly the night before? Was the shot squeezed into the busiest day of your week? Was your resting heart rate already creeping up before the appointment, the way it does when a cold is starting?

Then pick one lever for next time: book the shot on a lighter day, after a normal night's sleep, and leave the next evening free. Compare how you feel and what your numbers show the following year.

When to see a doctor

Contact a doctor or pharmacist if:

• tiredness or fever lasts longer than a few days or gets worse;

• the arm becomes increasingly red, hot, swollen or painful after the first two days;

• you notice palpitations, an irregular pulse or a resting heart rate that stays clearly above your normal;

• you have weakness or tingling in your legs or arms in the weeks after a vaccine;

• you're unsure whether to get your next dose.

Call 911 for signs of a severe allergic reaction, which usually appear within minutes to hours of the shot: trouble breathing, wheezing or hoarseness, swelling around the eyes or lips, hives, paleness, weakness, a racing heartbeat or dizziness (3). You or your doctor can also report a reaction to the Vaccine Adverse Event Reporting System (VAERS).

How to bring this up with your doctor

What to ask for, by name. "Is what I felt a normal reaction, or something we should note in my record?" If it was severe: "Should I get a different flu vaccine, or be observed longer next time?"

What to bring. The date and time of the shot, the vaccine name if you have it, your symptoms day by day, your temperature, and your resting heart rate and HRV for the week before and after.

What to say, out loud. "I was very tired for [number] days after my flu shot. My resting heart rate went from about [number] to [number] and came back on [date]."

If you're told it's nothing. That may be true, and that's good news. Say: "Thanks. Is there anything I should do differently next year, like the timing or the vaccine type?"

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How Welltory helps — and what it cannot do

The limits first. Welltory is a general wellness product, not a medical device. It does not diagnose, detect or monitor vaccine reactions, and it can't tell you whether your vaccine worked.

What it can do is help you compare the days after a shot with your own usual days.

Measure the same way each morning. Take a short spot reading with your phone camera and flash, or connect a compatible chest strap, Apple Watch or Samsung Galaxy Watch (7, 8). The help center suggests measuring in the morning, before coffee and breakfast, in the same position (9).

Tag the shot. Add "flu shot", "sore arm" or "tired". When the app flags a stress or recovery episode, it asks "What happened?" and you can tap, type or speak. My Patterns then shows which situations keep coming up around your stress and recovery (10).

Look at the next night. With an Apple Watch or Oura ring, sleep analysis shows how your heart rate behaved overnight (11); on Android, sleep analysis works through Health Connect (12). Remember that, on average, flu shots didn't measurably change heart rate in smartwatch data, so your numbers may look ordinary.

Keep a record. You can export your data as a CSV from the web app for an appointment (13).

How we made it

We based this article on a prospective smartwatch and questionnaire study comparing reactions to flu and COVID-19 vaccines; the CDC's flu vaccine safety guidance; a classic study of fever and heart rate; research on sleep and vaccine antibody response; and CDC information on colds. Search questions came from what people in the United States most often ask about feeling tired, heart rate and HRV after a flu shot. For the product section, we checked every function against the Welltory Help Center on November 8, 2026.

Made with AI tools, then edited and fact-checked by the Welltory team. See our Editorial & AI policy.

Jane Smorodnikova, co-founder of Welltory, built the methodology behind how we read physiological data.

Written by Tatsiana Yashyna.