This is not an obstacle for fast. The wives of the prophet Aisha and Ummu Salame reported that the Messenger woke up after dawn needing an ablution and had a bath while he was fasting. (Bukhari, Sawm/Fast, 22)
According to Islam, one who needs to perform major ablution can only wait for one prayer time. It is not permitted to wait any longer. You can sleep in the night without ablution but you need to get up at dawn and wash yourself in order to offer the morning prayer. If you miss its time, you need to do it very firstly when you wake up.
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