Yes it is Makrouh (Disliked) to sleep between Fajr and sunrise. Many authentic Hadeeths stated that it is recommended to be busy in that time ( between Fajr and sunrise) in prayers, Du'a, remembering Allah, reciting Quran and any useful deeds but not sleeping unless you are unwell or have another valid reason to sleep then. The benefits of being busy in that time in worship are too many including good health and more sustenance. Sleeping during that time might cause a diverse results on health and sustenance according to the Hadeeth. ( Manla Yahdharuhul Faqeeh, number 1439 and 1440)

Wassalam.