A certain inertia, tendency to sleep, indolence, unwillingness or inability to be strong for work or spiritual effort for long at a time, is in the nature of the human physical consciousness.

When one goes down into the physical for its change (that has been the general condition here for a long time), this tends to increase. Even sometimes when the pressure of the sadhana in the physical increases or when one has to go much inside, this temporarily increases—the body either needing more rest or turning the inward movement into a tendency to sleep or be at rest.You need not, however,be anxious about that. After a time this rights itself; the physical consciousness gets the true peace and calm in the cells and feels at rest even in full work or in the most concentrated condition and this tendency of inertia goes out of the nature.

Ref: Letters on Himself and The Ashram