what is the scientific reason behind it ? 
IN INDIA, from ancient time during sleep put head towards northside and feet towards southside is forbidden,used to be.And it is believe thatto do this, yamraj(death) has to call.
this is also known,that after death the dead body is placed in this direction for cremation.
Is this era of science,we all believe that,all this is superstition?
is this a superstition or is there any scientific fact behind it ?
Before Knowing this, we need to understand some points like earth (northern and southern hemisphere),magnetic line of earth,blood composition,position of heart,gravity and nervous system.
EARTH-
northern and southern hemisphere.
https://www.google.co.in/search
MAGNETIC LINES OF EARTH-
the magnetic line of earth are going from north to south pole.
Blood composition -
as we all know about haemoglobin which contain iron(Fe)
position of heart-
above 1/2 of the body
SCIENTIFIC REASON
If you place your head towards northside and sleep 5 to 6 hours then magnetic pull will cause pressure on your brain because blood contain iron(Fe2+) having magnetic properties.
along with age,the blood vessels become weak, you could have hemorrhage and paralytic strokes.when you sleep horizontally,your pulse rate immediatel drops lower blood circulation in brain then more comfertably you can sleep.if the blood circulation higher in brain then you could get up agitated.
if you sleepeveryday in that direction you are asking for trouble,After sleeping you will not feel good.
BEST direction is to sleep as explain in figure.
IN NORTHERN HEMISPHER
Put head in south and feet in north direction
IN SOUTHERN HEMISPHERE
Put head in north and feet in north
Placing dead body in south side for cremation have a scientific reason
according to our ancient scientist magnetic lines affect our brain and brain blood circulation that may it stimulate brain activity, consequently we are trying to save a life at last.so there is scientific reason not superstition.
source of poster https://www.google.co.in/search