It does sound strange, right!! I am not sure actually if it a verified scientific belief but I am sharing this with you because I feel it might be true to a certain extent. But, not necessarily for all. Sleeping is one of the most important activities in our life. If you know, we sleep for more than 1/3rd of our lives! I know that its a lot. So, definitely the way we sleep would have some or the other effect on our body.
There are a lot of ways people sleep and the most common way is to get curled up and sleep on the side or stomach. This might be a great way to get deep sleep but definitely not good for your height. Because, it severely restricts your spine from getting relaxed and comfortable. And, if your spine is not straight, you would look almost 2 – 3 inches shorter than your actual height. You know that when you get up in the morning, you are an inch or two taller than your normal height and when you go to bed at night, you lose those inches. This is because of the gravitational pull on your spine. Now, as to the number of inches you lose, it also depends greatly on you.
So, if you want to increase your height and look taller, the best way is to sleep flat on your back. Or, no matter how you sleep, just make sure that your spine is straight. You can tuck a pillow in between your knees if you sleep on the side or under your knees if you sleep straight to prevent exerting pressure on your spine. I have noticed that if I sleep on my back, I tuck one of my legs under the other and if I sleep on my side, I tuck my hands in between the knees. This is probably just an instinct of the body to avoid strain on the spine. Not using a pillow is also a good thing for your spine but many people are used to using a pillow so use a small pillow instead of a high one. It will give relief to your neck and avoid uncomfortable postures which you might develop while using high pillows. And, regrading the mattress, use one which is not too soft and not too hard. Rather, choose one which is a bit towards harder side instead. It is good for the health of your spine. And, believe me, you do not want to get back problems ever in your life. I am sure all people suffering from back problems will agree with me. If you never encountered one, you are lucky and strive to remain that way. And, console yourself that it sure makes you look taller too.