What is drowsiness? By making the cause of drowsiness its point of departure, the mind is agreeable to the positive, negative, indifferent, timely, untimely, appropriate, and inappropriate. Drowsiness is related to bewilderment-erring. Its function is to become the basis of slipping away from what must be done.It is a strong subjective attitude in which perceptive organs which operate on the object are made to turn helplessly inward because of being preoccupied with the heaviness of body, little excitement, weariness, and a general haziness.-Abhidhdarmasamuccaya
Here, the statement 'timely and untimely' is elaborated by Nagarjuna (Suhrlleka, 38) who says,
Oh great beings! During the day and evenOne must understand that the time to sleep is during the night. The time during the forenoon, afternoon and daytime, not being the time to sleep, is the time for striving assiduously to do positive actions.
During the night, the time after sunrise and sunset,
Even while asleep, let the time not be fruitless!
Don't let inspection slip away during those times.
The statement 'appropriate and inappropriate' is employed for the sake of understanding that during the night sleep is appropriate for increasing the ability of the body to attend to positive tasks; but sleep which is disturbed by the emotions, even if it be during the night, is inappropriate. The reason for this can be known from the texts quoted above.
Its activity is explained as 'slipping away from what must be done' because sleep has two aspects -- the positive and negative. The negative aspect of sleep which is emotionally tainted makes one dread positive tasks which must be done. The positive aspect of sleep is explained in detail in the Bodhisattvabhumi, but since it would require too many words to quote, I shall omit it.