WinSleep by MollieSoft is a Windows utility that keeps your computer asleep when it's not busy; a timeline shows when your computer was asleep or awake over the last several days. It is compatible with Windows 8.1 or Windows 10. More info is available at www.molliesoft.com/WinSleep, including screenshots and a download link for the 30-day free trial. A permanent license for WinSleep costs $9.95.
