What is 'Microsleep' (Sekundenschlaf) and what are its physiological warning signs?

1. A deep meditation technique practiced by professional race drivers
2. An involuntary lapse into sleep lasting from a fraction of a second up to several seconds caused by acute fatigue; warning signs include heavy eyelids, frequent yawning, line drifting, and missing road exits
3. A feature of electric vehicle battery sleep mode
4. A brief pause in engine ignition during gear shifting