Hour by Hour

A fast unfolds in stages, but the timing is approximate:

• 4-8 hours - insulin falls and you start shifting from sugar to fat for fuel
• 12 hours - stored fuel use increases, and hunger often comes in waves
• Longer fasts - autophagy may increase, but the exact human timing is not well established
• 24 hours and beyond - the body relies more heavily on stored fat, and medical caution matters more

Autophagy is your cells' built-in cleanup crew, breaking down worn-out proteins and recycling them. Fasting may stimulate it, but no everyday schedule guarantees a precise switch-on hour.