Your body regulates its responses to heat, cold, stress, infection, and other conditions without any conscious thought. You sweat to cool the body when you get overheated, for example, but you don’t have to think about it. And when you get cold, you shiver automatically.

A shiver is caused by your muscles tightening and relaxing in rapid succession. This involuntary muscle movement is your body’s natural response to getting colder and trying to warm up.

Responding to a cold environment, however, is only one reason why you shiver. Illness and other causes can also make you shake and shiver.

Read on to learn more about shivering.

There are many things that can make you shiver. Knowing what can trigger a shiver will help you know how to respond.

Cold environment

When the temperature drops below a level your body finds comfortable, you may start to shiver. Visible shivering can boost your body’s surface heat production by about 500 percent. Shivering can only warm you up for so long, though. After a few hours, your muscles will run out of glucose (sugar) for fuel, and will grow too tired to contract and relax.

Each person has their own temperature at which shivering starts. For example, children without much body fat to insulate them may begin shivering in response to warmer temperatures than an adult with more body fat.

Your sensitivity to cold temperatures can also change with age or because of health concerns. For example, if you have an underactive thyroid (hypothyroidism), you’re more likely to feel cold more acutely than someone without the condition.

Wind or water on your skin or penetrating your clothing can also make you feel colder and lead to shivering.

After anesthesia

You may shiver uncontrollably when anesthesia wears off and you regain consciousness following surgery. It’s not entirely clear why, though it’s likely because your body has cooled considerably. Operating rooms are usually kept cool, and lying still in the cool operating room for an extended period of time can cause your body temperature to decrease.

General anesthesia can also interfere with your body’s normal temperature regulation.

Low blood sugar

A drop in your blood sugar levels can trigger a shivering response. This can happen if you haven’t eaten for a while. It can also happen if you have a condition that affects your body’s ability to regulate blood sugar, such as diabetes.

Low blood sugar can affect people in different ways. If you don’t shiver or tremble, you may break out in a sweat, feel lightheaded, or develop heart palpitations.

Infection

When you shiver, but you don’t feel cold, it could be a sign that your body is starting to fight off a viral or bacterial infection. Just as shivering is your body’s way of warming up on a chilly day, shivering can also heat up your body enough to kill a bacteria or virus that has invaded your system.

Shivering can actually be a step toward developing a fever, too. Fevers are another way your body fights off infections.

Fear

Sometimes, shivering has nothing to do with your health or the temperature around you at all. Instead, a spike in your adrenaline level can cause you to shiver. If you’ve ever been so afraid you started trembling, that’s a response to a rapid rise in adrenaline in your bloodstream.

You probably don’t remember a time when you didn’t or couldn’t shiver. That’s because the only time in your life when you don’t shiver is at the beginning.

Babies don’t shiver when they’re cold because they have another temperature-regulation response. Babies actually warm up by burning fat in a process called thermogenesis. It’s similar to how hibernating animals survive and keep warm in the winter.

If you see a baby shivering or shaking, it could be a sign of low blood sugar. Your baby may simply be hungry and in need of energy.

In older adults, a tremor may be mistaken for a shiver. There can be several causes of tremor, including Parkinson’s disease.

Some medications, such as bronchodilators used for asthma, can also cause shakiness.

As you get older, you may also become more cold sensitive. This is due, in part, to a thinning of the fat layer under the skin, and a decrease in circulation.

Shivering may be a symptom of an underlying condition, so you shouldn’t ignore it. If you feel especially cold, and putting on a sweater or turning up the temperature in your home is enough to warm you up, then you probably don’t need to see a doctor. If you notice that you’re getting colder more often than you once did, tell your doctor. It may be a sign you should have your thyroid checked.

If your shivering is accompanied by other symptoms, such as fever or other flu-like complaints, then see your doctor immediately. The sooner you identify the cause of your shivering, the sooner you can start treatment.

If you notice a tremor in your hands or legs that’s clearly not a cold-related shiver, report these symptoms to your doctor.

The right treatment plan for your shivering and other symptoms will depend on their underlying cause.

Cold environment

If your shivering is a response to chilly weather or wet skin, then drying off and covering up should be enough to halt the shivers. You may also need to set your home’s thermostat to a higher temperature if age or other conditions are making you more sensitive to the cold.

Make a habit of bringing a sweater or jacket with you as you travel.

Infection

A virus usually needs time to run its course. Often, the only treatment is rest. In some serious cases, anti-viral medications may be appropriate.

If you have a fever, gently sponging your skin with lukewarm water can help cool down the body. Be careful not to put cold water on your skin, as it can cause you to shiver or make your shivering worse.

A bacterial infection will usually need antibiotics to knock it out completely.

If you get the chills because of an illness, be careful not to overheat with too many blankets or clothing layers. Take your temperature to make sure you’re not running a fever. Lighter covering may be best.

Low blood sugar

Eating a high-carb snack, such as a peanut butter sandwich or a banana, can often be enough to get your blood sugar levels back on track. In general, you don’t want to go too long without eating. This is especially true if you’re prone to drops in your blood sugar or have trouble keeping your blood sugar levels in a healthy range.

If this a problem, be sure to keep a granola bar or similar snack handy at all times. That way you’ll have something on hand to eat if you feel your blood sugar dropping.

Postsurgery

Usually, a few blankets tucked around you after surgery is enough to warm you up and put an end to the shivering. If you’re uncomfortable or concerned about the shivering, let your nurse or doctor know.

When shivering is a response to feeling cold, grabbing an extra blanket or pulling on a sweatshirt can usually still your muscles and warm you up. A hot cup of tea or coffee can also help.

If you’re sick, remember that shivering may be the start of a fever, so be careful not to get overheated. And if you notice that you, your child, or an aging parent are trembling, but it doesn’t seem to be caused by one of the traditional causes of shivering, notify a doctor. Shivers, chills, shakes, and tremors are all symptoms of something, so take them seriously.