Sleeping during the day

It [sleepiness during the day] may possibly be due to the attempt of the higher consciousness to descend then. It sometimes produces this effect of sleepiness on the body, for the physical attempts to go inside to meet the descending consciousness and if it is not accustomed to enter into one of the higher samadhis on such occasions, the going inside translates itself to the physical as sleep. The exercise may have contributed, of course, by its reaction on the body.

Ref: Letters on Yoga – IV