Generally, it is always hot in the afternoon.
Higher the temperature, higher is the rate of transpiration. 
This may result in excess of water loss from the plant body due to excess of transpiration.
So, to avoid the loss of water through transpiration, plants generally wilt in the afternoon.

When the unfavourable conditions go away, plants no more wilt and stay open.
All the night and in the mornings, the temperature is not very high. So, no excess of transpiration occurs.
Therefore, plants do not wilt. Rather, they stay fresh and turgid.
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