Sri Aurobindo
Letters of Sri Aurobindo
Volume III - Part 3
It [the tendency to fall asleep during meditation] is a common obstacle with all who practise Yoga at the beginning. The sleep disappears gradually in two ways – (1) by the intensifying of the 
force of concentration, (2) by the sleep itself becoming a kind of swapna samadhi in which one is conscious of inner experiences that are not dreams (i.e. the waking consciousness is lost for the time, but it is replaced not by sleep but by an inward conscious state in which one moves in the supraphysical of the mental or vital being).