It was the practice of the Prophet ﷺ and his Companions (Radiyallahu Anhum – may Allah be pleased with them) not to sleep after the dawn prayer, the first of the five daily prayers, as it is a time of blessing. He would read some parts from the Qur’an and reflect on the universe and its Creator at a time when concentration is at its peak and the mind is clear of daily worries. Once the dawn broke, the Prophet ﷺ would pray and then speak with people who stayed behind (in the Masjid) for some time. Therefore, some of the early Muslims disliked the idea of sleeping immediately after dawn because being awake at this time was encouraged.
Sleeping for a short period of time during the day called “Qaylulah” (nap, siesta) is in accordance with the Sunnah of the Prophet ﷺ. It is from mid-morning to just past noon. This kind of sleep is part of the Sunnah since it allows a person to rise at night to pray. It is also the custom of the people of the Arabian Peninsula to rest from work at noon when it is intensely hot, so it has further strengthened this practice of the Prophet ﷺ.
This sleep increases both life and sustenance. For half an hour of “Qaylulah” is the equivalent of two hours’ sleep at night. Recently, this type of sleep has been strongly advised for health reasons and for enhancing daily performance. And Allah knows best.
[excerpted from onislam.net with slight changes]