It is not forbidden to sleep at anytime in the day or night. However, it would be contrary to the blessed Sunnah of Nabi (Sallallaahu Alayhi Wasallam).
It is established that Nabi (Sallallaahu Alayhi Wasallam) would not sleep after Fajr Salaat, but would remain in the Dhikr of Allah, the tilaawat of the Qur’aan, etc. until Ishraaq, after which he would engage in other chores. After Zawaal, however, a siesta is recommended.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai