No - some helpers are worked 24 hours a day!! They sleep with the baby and do night shift, then do all the house work and cooking during the day, as well as marketing and childcare. My helper generally is up around 7am and finishes work around 8pm, but she has plenty of down-time during the day. My husband and I have recently been swimming a few evenings a week, and we occasionally have a date night, so our helper will help with the kids until later. Generally they are asleep by 8-9pm though so it's not even that much extra work. If we went away for an entire weekend, I might think of getting her something as a thank you (plus we would be legally obligated to give her another day off if we required her to work on a Sunday) but extra money for evening work is not necessary, and in my opinion, I wouldn't want her to expect that!! It's just a part of her job, like Cara said.
(just wanted to add, I think it's inhumane if they work 24 hours a day - I personally feel as though in the VERY least, they should have 8-10 hours with absolutely no work commitments whatsoever)