They used to sleep little by night.
Rarely did they sleep the whole night.
They slept but a little at night
The used to sleep but little at night
They were little from the night what they (spend in) sleep
They cut their sleeping at night short to worship their Lord
They used to sleep only a little during the night
They slept but a little of the night
They used to sleep at night only for a short while
They were in the habit of sleeping but a little by night (for their being occupied in God's worship)
They used to sleep but little by night (invoking their Lord (Allah) and praying, with fear and hope)