An internal clock was an individual's automatic time regulation system based on daily cycles.
In 2365, six-hours-ahead-Jean-Luc Picard's internal clock realigned as time progressed closer to his origin point. (TNG: "Time Squared")
In 2367, measurements at the cellular level lead Beverly Crusher to conclude that the crew of the USS Enterprise-D had lost consciousness for longer than thirty seconds, as was previously believed. (TNG: "Clues")