Workers who are required to be available to perform work as and when called upon are entitled to the National Living Wage (NLW) and National Minimum Wage (NMW) for the entire shift if they are deemed to be “working”. This remains the case, even if they are permitted to sleep during that shift.
The Government asks the Low Pay Commission (LPC) to monitor and evaluate the impact of the National Minimum and National Living Wage rates. The LPC carry out extensive research, consultation and analysis across a range of sectors to inform its wage rate recommendations.
The LPC autumn 2016 report, summarises its consultation with business stakeholders on the impact of the NLW and NMW. The report is available here - https://www.gov.uk/government/publications/national-minimum-wage-low-pay-commission-autumn-2016-report
The Government also produces analysis of the NMW/NLW rates in its Impact Assessment, this projected that around 126,000 workers are affected by the April 2017 rate rises (p79). Available here - http://www.legislation.gov.uk/uksi/2017/465/impacts