I agree with gooseeggs points. I think that an overnight for a nanny or childminder depends very much on what the child in question is like.
Is it an overnight on top of a full day with another full day afterwards?
Is it an overnight in your own home, ie child comes to you?
Is it a child over the age of 4/5 and night potty trained?
Does the child wake during the night?
Are you being expected to sit up all night? ie not go to bed at all?
An overnight 6pm-8am with child sleeping in spare bed, you in your own bed, no wake up calls and child going home at 8am the next day isn't that much hard work surely?
An overnight with a baby who still wakes 2 or 3 times a night needing milk/nnappy changes is different.
Yes nightshift can be paid more than daywork but not always, often people make a choice about 'when' they want to work. If the idea of maybe having to get up once to a mindee, or having to share your space from 5pm-8pm with another child isn't for you then maybe you don't offer to do night care.
Our nanny has said she would see an overnight as like another days work each has ups and downs. Daytime is harder cos there's alot more to do with an active toddler - ie games, meals, running in the park, cleaning etc. Nighttime could be harder because its a night away from her own bed and she may have to get up to give him a drink, on the other hand from 7pm - 11pm is her own time to sit and watch TV, read a book, go online etc and still get paid for it.
Mind you I wouldn't expect her to do 8am-5pm, then 5pm-8am, then 8am-5pm all in a row!
Last time she did overnight for us she came in a bit late and worked 10am-5pm, then had time off to go and get pizza or cook something in our kitchen while I bathed him, then she took over at 6.30pm and put him to bed as we went out. She finished at 8am the enxt day and went home.