Internal clock may refer to:

• Circadian clock, a biochemical mechanism that oscillates with a period of 24 hours (in accordance with the day-night cycle); driver of the circadian rhythm
• de Broglie internal clock, hypothetical clock in an electron, constituting part of the mechanism by which a pilot wave guides a particle
• Electronic oscillator, an electronic circuit that produces a periodic, oscillating signal, often a sine wave or a square wave