The cause of severe leg cramps at night is usually lack of activity during the day and loss of blood circulation through the leg.
You could have poor circulation in your leg or you could have strained your leg muscles during the day or it could be the position your sleeping in.
Severe leg cramps at night are usually harmless and nothing to worry about but if they continue you might want to see a doctor about it.
You could also have restless leg syndrome that is causing the leg cramps to happen at night.
To prevent leg cramps at night try stretching your legs and leg muscles before going to bed.
Stay hydrated by drinking plenty of water and get plenty of exercise during the day.
Dehydration can cause leg cramps.
If you're dehydrated and you experience the leg cramps often when dehydrated that could be one of the signs that you're dehydrated and if so you need to hydrate yourself by drinking some water or other healthy drinks.
You could also have just overused the muscle or strained the leg muscle and that could have led to leg cramps.
If the leg cramps go away once you're hydrated then it's likely that you were dehydrated that caused the leg cramps but if not then it could be something else.
Poor blood circulation as well as a lack of vitamins such as including thiamine (B1), pantothenic acid (B5), and pyridoxine (B6) can be a cause of leg and muscle cramps as well.
If the leg cramps continue or don't go away on their own it would be best to see a doctor or go to urgent care to get checked out.
Usually the leg cramps are nothing serious but it won't hurt to have it checked out just in case.