A sleep function capable of putting a fixed high-priority thread to sleep within a time-window is disclosed. After a sleep request has been made by a fixed high-priority thread via the sleep function, a determination is made whether or not the fixed high-priority thread is awoken before a requested sleep duration under the sleep request. If the fixed high-priority thread is awoken before the requested sleep duration, the number of tasks for the fixed high-priority thread to perform is increased in order to delay the start sleep time of the fixed high-priority thread from a point within a first time-window in which the sleep request was made to an end boundary of the first time-window.