Hypnic jerks, also known as sleep starts, are involuntary muscle contractions that happen when falling asleep. They can range from mild twitches to strong spasms that wake you up. Around 70% of people experience them, especially during the transition between wakefulness and sleep.

“Sometimes, your brain sends an unintended signal to the muscles, causing them to contract suddenly.” This happens when the brainstem and thalamus, which control sleep transitions, miscommunicate with muscles. The brain might misinterpret muscle relaxation as a loss of balance, triggering a sudden jerk — possibly an evolutionary reflex to prevent falling.

Theories suggest hypnic jerks are either leftover survival mechanisms from early humans or the brain’s way of correcting perceived imbalance. Factors like stress, caffeine, intense exercise, and sleep deprivation can increase their frequency.

“While they can be frustrating… they don’t pose any serious health risks.” Most hypnic jerks are harmless, but if they severely disrupt sleep, consulting a doctor is recommended.

To reduce hypnic jerks, practice relaxation techniques, avoid stimulants before bed, exercise earlier in the day, and maintain a consistent sleep schedule.