An overnight is the eight-hour booking, one fixed fee for one client with one therapist and more for couples or two therapists, always confirmed with half the fee in advance.
On the house ladder, overnight is a fixed format rather than a loose description. It means eight hours, beginning at an agreed time in the evening and ending eight hours later, at one fixed fee for a single client with one therapist. It is not three hours that happen to fall late, and it is not an open-ended night: the start and finish are named in the confirmation.
The format exists because the hourly rungs lose their logic past a certain length. For one client with one therapist, each hour after the third adds the same again as the first, so a straight hourly count toward a full night would pass the fixed night fee by the seventh hour. The eight-hour figure reflects a night given over to one client rather than an arithmetic sum. The full set of rungs is laid out in the duration ladder entry.
An overnight always carries a deposit, whether it is booked a week in advance or for the same evening. It is half of the fee, whatever that fee comes to, paid by card or another accepted method once the booking is accepted. Whatever is left of the fee is settled once she is at the door. If plans change, the notice decides what is kept as fee already earned: nothing more than 24 hours before the night begins, half the deposit inside that time, all of it once she has set out, and the whole deposit back if another client takes the night.
The night can also be booked for more than one person on either side. For a couple sharing one therapist, the night is charged at twice that. One client keeping two therapists through it pays more again, and a couple with two therapists pays twice that. The deposit is half of whichever row applies.
It is worth booking an overnight earlier than a single hour. Therapists who take them plan the following day around them, and on busy dates the evenings fill first. The overnight service page describes the format at more length, and the booking page sets out the steps from first message to confirmation.