An hourly-paid worker should be paid at least the National Minimum Wage for every hour that you are contracted to work. If you are required to remain at the workplace (in your case, the service user’s home) for the whole night shift, so that, for example you can be disciplined by your employer if you leave the premises without permission or if you don’t turn up for your shift, then you will be working for the entire shift and should be paid for it.

The fact that the work you are doing is 'sleeping', or that you are rarely disturbed, does not stop it being 'work'!