CPS employed on-call guards to provide security at construction worksites. Part of each guard’s day was spent on active patrol. Each evening, guards were required to remain on call and on premises at the worksite to respond to disturbances should the need arise. By written agreement, on-call guards were required to reside in a trailer at the worksite. An on-call guard had to notify a dispatcher and indicate where he or she would be and for how long. If another employee was not available to provide relief, the guard had to wait onsite until the reliever arrived. If relieved, guards had to be accessible by pager or radio phone and to stay close enough to the site to return within 30 minutes. The guards were paid hourly for the time spent patrolling the worksite, but received no compensation for on-call time unless an alarm or other circumstance required that they conduct an investigation or they were waiting for or had been denied a reliever. In this putative class action, both sides sought declaratory relief as to the lawfulness of CPS’s on-call compensation policy. The trial court granted plaintiffs’ motion for summary adjudication of their declaratory relief claim, concluding the on-call hours constituted compensable “hours worked” within the meaning of the applicable wage order. The Court of Appeal affirmed in part and reversed in part, and the California Supreme Court concluded in this opinion that plaintiffs’ on-call hours constituted compensable hours worked and that CPS could not exclude “sleep time” from plaintiffs’ 24-hour shifts.