It’s quite common that meditation can be so calming and relaxing that it draws us out of alertness into sleep.
This is no problem if you’ve been having trouble sleeping or lack rest.
The question in meditation is how to stay alert in this calmness and ease we so often associate with sleep.
The secret ingredient is alertness and inquisitiveness about this calmness and ease. This brings a sense of aliveness and enthusiasm within this ease.
The Every Day Yogi 