Plain English Meaning
Your head keeps nodding forward on its own and snapping back up. You are not choosing to do it. It is happening because you are so tired that you are falling asleep for a second or two at a time without realising it.
Full Definition
A physical sign of advanced fatigue in which the head drops forward and jerks back without conscious control, indicating that the person is briefly losing muscle tone as they slip in and out of micro-sleep.
Context Anchor
Seen in fatigue discussions, especially when identifying warning signs that a pilot or instructor is no longer safely alert.
Why Pilots Care
Signals dangerous fatigue that can cause loss of aircraft control and requires immediate corrective action such as landing or handing over .