You must be paid for all the hours you are contracted to work. When you are required to sleep over in a service user’s home, you are still working. The fact that the work you are doing includes sleeping, or that you are rarely disturbed, does not stop it being 'work'!
Your rate of pay depends on your employment contract, but as a minimum, your employer must comply with National Minimum Wage laws. Your average basic hourly rate over your pay reference period, including time spent sleeping over at the service user’s home, must be at least the National Minimum Wage rate. Otherwise your employer will be breaking the law.
Your pay reference period is based on the pay arrangements where you work. It will be a day if you are paid daily, a week if you are paid weekly and a month if you are paid monthly.