By Caroline Ryan
BBC News Online health staff
Lying in bed, that peculiar feeling in your chest is probably heartburn - especially after a rich festive feast.
Changes in heartbeats may not be down to ill health
But in the darkness, you may begin to fear you are having a heart attack.
Or you may be perturbed by a thudding feeling in your chest, or the strange sensation of your heart beating through their stomach.
How the heart works is a mystery to many, and any change can cause concern.
But experts say the explanations for these peculiar sensations are very unlikely to be serious.
In the small hours, a fluttery feeling in your chest can be a disturbing feeling.
But ectopic heartbeats, where the heart feels like it has an extra beat, are very common.
They are more common in people who have heart disease, but most people have at least one every 24 hours.
Ectopic heartbeats usually feel like a thud in the chest, or an irregular heart rhythm.
They can be especially noticeable when lying in bed when you can 'hear' your heart rhythm.
Cathy Ross, a cardiac nurse at the British Heart Foundation, told BBC News Online: "Quite a lot of people do have extra heartbeats
"In themselves, they're not dangerous and don't damage the heart.
"However, if there are symptoms such as feeling unwell, they need further investigation to find out what's going on."
In those circumstances, doctors will carry out checks including an electrocardiogram to see if there is an underlying problem."
While extra heartbeats are usually not a problem, having a slow heartbeat may be.
In athletes, a slow heartbeat is normal and a sign of rude good health.
The heart is a muscle and because they are so fit, it doesn't have to work so hard to pump blood around the body.
When people are feeling sick or nauseous it is also normal for the heartbeat to slow down.
But a slow heartbeat in those who aren't super-fit can be a sign there is a problem.
Cathy Ross says what is seen as abnormal depends on what someone's normal heart rate was.
"If 56 is someone's normal heart rate, it could be said to be bradycardic (slow), but it might be normal for that person, so it wouldn't be anything to worry about.
"But if someone's heart rate was normally 72, you would be looking at why it had slowed."
Some drugs could have the effect of slowing the heart down, or it could be caused by heart failure or an electrical disturbance in the heart.
Even body shape can lead to people believing they could have a problem.
Some people who are very thin apear to show signs of having an aortic aneurysm.
An aortic aneurysm occurs when there is a swelling in the large artery leading out of the left side of the heart and supplying the whole body with blood.
If an aneurysm bursts, it can cause severe internal bleeding which can be fatal.
But slender women can show apparent signs of aneurysms, even though they are not at risk, because of their figure.
Because they are thin, they can feel the pulse of their aorta through the aneurysm.
This is because their aortas are closer to their stomach.
All aortas can be felt pulsating from the front and back of the body - the pulse is just more clearly felt in thinner women.
Vascular surgeon Mr Andrew May said this can lead some patients - and some inexperienced doctors - to think they have an aortic aneurysm.
Mr May said experienced doctors were able to tell the difference: "The test is to feel down the side, spreading your fingers out."
He warned people should see their doctor, even if they were thin, because not everyone displayed symptoms of aneurysms.
"People very often don't show any symptoms at all, the vast majority don't.
"That's one of the reasons we're pressing to have a screening programme."
Men are eight times more likely to have an aneurysm than women.