Q:

Why not sleep with your head facing North direction?

People usually avoid sleeping with head facing to the North. What I heard is that doing so will bring death. Is this true?
What is the reason behind this?

A:

Religious Reason
The south corner is considered as a direction of Death. It also said that the Yamalok ( hell ) is located in the south of the universe and beneath the earth.
Normally when a person dies, his/her body should be kept in a position like head towards north. This is one of the strong reason that people avoid sleeping head towards north direction. For more information read Narak article from Wikipedia.
Scientific Reason
Science says that it is because the human body has its own magnetic field (Also known as hearts magnetic field, because of the flow of blood) and Earth is a giant magnet. 
When we sleep with head towards north, our body's magnetic field becomes completely asymmetrical to the Earth's Magnetic field. 
That cause problems related to blood pressure and our heart needs to work harder in order to overcome this asymmetry of Magnetic fields. Apart from this, another reason is that our body has significant amount of iron in our blood. 
When we sleep in this position, iron from the whole body starts to congregate in our brain which can cause a headache, Alzheimer’s Disease, Cognitive Decline, Parkinson disease and brain degeneration. All these things don't happen in one or two single night but it happens if the person has a daily habit of sleeping in a north direction.

A:

The reason we don't keep the head in the north and legs in the south is because south direction is associated with Yamalokam. From Wikipedia

Naraka (Sanskrit: नरक) is the Hindu equivalent of Hell, where sinners
  are tormented after death. It is also the abode of Yama, the god of
  Death. It is described as located in the south of the universe and
  beneath the earth.

So, in Hindus, we lay down a person with his head to north and feet in south when they die. Also if you know, we always hang dead peoples photo frames in the south. Hence, its considered that sleeping in this direction may bring trouble, distress etc to an individual. 
On the other hand, a person can sleep in an opposite direction for example, he can keep his head in the south and feet facing north. This sleeping position is also suggested in Vaastu Shastra

One should sleep with head towards South, East or West and never in
  north.

You can refer a post below which explains cultural as well as scientific reasons (though as pointed out by bjb568 there are scientific reasons towards this and yes, the article has an alternate story as well)

