The jolt feeling is most likely caused by the adrenaline that your body makes when you encounter whatever scares you in the dream. When we get angry or scared, we get an adrenaline rush that helps us decide whether we fight or run (called "fight-or-flight").

Adrenaline is an essential hormone that helps increase blood flow to our muscles, our heart rate, our blood sugar, and the size of our pupils. All of these changes help our bodies become temporarily better able to respond to our situation. Adrenaline is also described as a stress hormone because it is secreted when we're under stress, and it wakes us from sleep because our body is sending us the signal to respond and not stay asleep!

Another hormone released under stress is cortisol, and this may also be part of what wakes us up. In fact, our bodies cannot go to sleep at night if there is too much cortisol or adrenaline, and a large quantity of cortisol is released in the morning to wake us up naturally.

I am not sure we really understand dreams yet, but they definitely can cause adrenaline rushes. Generally, if you wake up and your heart is pounding, that's because you're rushed with adrenaline.

There are at least two possibilities here. One is that you are getting scared by the dream and getting a shot of adrenaline. Scary things in dreams can cause a release of adrenaline just like scary things when you’re awake. And what’s scary in a dream might not really be scary.

There’s another possibility. When you dream, your brain has to sort of disconnect from the rest of your nervous system. Normally you would get incoming signals from all your sensors, like the ones in your skin and ears. But in sleep, your brain responds to the signals that it creates. You think you hear, see, and feel things that are only happening in your brain. Your brain also has to disconnect from the nerves that control your muscles so that when you are fighting that dragon in your dreams, you’re not running around the house knocking things over. The nerves aren’t really unplugged, the signals just don’t get sent, or a tiny bit of the signal get sent, so you only hear loud noises, or your leg twitches instead of you running.

There’s an area in your brain stem called the reticular formation that is in charge of disconnecting and reconnecting the signals as you fall asleep and wake up. Usually you don’t even notice this, but sometimes the timing is slightly off, so when you start to fall asleep, you feel like you’re falling and wake up scared. Other times you might be awake but feel paralyzed for a few moments before the signals get going again. Some people hear loud bangs or roars, or see flashes of light just as they wake up. What’s happening to you could be like that. It’s called “exploding head syndrome.” Sounds scary! But there’s nothing dangerous about it.

Sleepwalking is another problem that can happen when the reticular formation isn’t quite working right. The person is somewhere between sleeping and waking and can hurt themselves. A prescription sleeping drug was pretty well known for causing this problem. Often people ate in their sleep and didn’t remember it in the morning. Sometimes people taking this medication would fall down stairs, or even drive. That’s scary.

What do you think is going on? Which part of what is happening to you seems to match these possibilities best?

Thanks for asking.

While sleeping, people sometimes experience what is called a hypnagogic jerk, an involuntary muscle spasm, that jolts you awake. This may be what you are experiencing. These are often correlated with dreams or an association of falling. It is quite common and most people experience them occasionally. It is not generally known what the underlying cause is, but it is possible that stress, fatigue, caffeine, or other medications that act as a stimulant can increase the frequency of hypnagogic jerks. If these jolts are causing significant disruption of your sleep, you should talk to a sleep specialist.