I think it's a primal, safety, security thing.

the darkness and the quietness of night is much more likely to make a calm birth space for the mother. There is too much going on during the day and the daylight for the body to really engage in labour.

I read something about it last pregnancy and it made so much sense. I haven't come across it again this time around but I have been looking for the passage as it really resonated with me last time.

Labouring though the night can be very good, you can make real progress! Just make sure you do try and get some sleep... and if things slow down during the day again - definitely get some sleep!!!