There has been some to-ing and fro-ing over this. Current official guidance says that, if you specify hours when people may sleep in their employment contract, and you provide them with suitable sleeping facilities, then you do not have to pay them for those hours, except insofar as any work is done in them. If you do not specify any sleeping time, however, an Employment Tribunal might require you to pay for the whole of the time that the worker is at work.
The European Commission has obtained agreement that on-call time will be defined much more precisely, to differentiate it from working time and rest time. However, this change will have to be implemented through legislation in the UK. The proposal is to define 'active' on-call time, which will count as working time, as time which workers are required to spend at your premises, but during which they are not required to perform any duties. These changes are unlikely to become law before 2010.
