After 20 hours without sleep your brain functions as if it had reached the legal blood alcohol limit in the State of California. (Per California's driving under the influence or DUI laws, it is illegal to operate a motor vehicle with any of the following blood alcohol concentration or BAC percentages: 0.08% or higher―21 years old or older operating a regular passenger vehicle. 0.04% or higher―when operating a commercial vehicle.) There are good reasons for that. Fatigue is believed to have contributed to both the Exxon Valdez and space shuttle Challenger disasters. Motorist sleepiness accounts for 33 percent of traffic accidents. In some countries, sleep deprivation accounts for an estimated $16 billion in annual medical costs.
David K. Randall points out in Dreamland, that within the first 24 hours of sleep deprivation, the person’s blood pressure begins to rise, then metabolism processes start to go haywire, resulting in an uncontrollable craving for carbohydrates. Soon the body temperature drops and the immune system gets weaker. If this goes on for too long, there is a good chance that the mind will turn against itself, triggering brain phenomenon in which the person experiences visions and hears phantom sounds akin to a bad acid trip.
(For more information see “Age-Proofing Your Brain” by Taylor and Briggs.)