Whole-day fasting involves regular one-day fasts. The strictest form would be Alternate day fasting (ADF). This involves a 24-hour fast followed by a 24-hour non-fasting period.[2] The 5:2 diet allows the consumption of 500–600 calories on fasting days.[3][4] Time-restricted feeding (TRF) involves eating only during a certain number of hours each day.[5] A common form of TRF involves fasting for 16 hours each day and only eating during the remaining 8 hours, typically on the same schedule each day.[6] A more liberal practice would be twelve hours of fasting and a twelve-hour eating window, or a stricter form would be to eat one meal per day, which would involve around 23 hours of fasting per day.[7]