Sleep

Sleep is a derived statistic in Fallout 3, Fallout: New Vegas and Fallout 4's respective Hardcore mode and Survival mode, and Fallout 2d20.

Characteristics
Sleep is a consequential statistic of the total time spent awake: the longer one is awake, the higher the sleep value becomes. Beverages containing caffeine will reduce this value, as will sleeping in a bed.

Fallout 3
In Fallout 3, Sleep modifies Hit Points.

Fallout: New Vegas
In Fallout: New Vegas, sleep is measured in Hardcore mode as SLP in the Pip-Boy 3000. Sleeping in an owned or rented bed will give the Well Rested bonus for 12 hours. As sleep deprivation worsens, stats will be negatively impacted, starting with Agility. Not sleeping for two weeks equals death. Each hour of sleep will reduce the SLP meter in the Pip-Boy 3000 by 58.

Bugs
Sometimes the SLP meter on the Pip-Boy 3000 can get stuck on zero, and in fact will not display any number below the indication arrow. This is normally caused by traveling through canyon wreckage, which sets all hardcore meters to zero. During this time the PC will not advance into sleep deprivation. This bug may be fixed by finding a bed and sleeping for any amount of time. After the PC awakes, the gauge will display the number and resume its normal function.

Fallout 4
In Fallout 4, Sleep modifies Fatigue in Survival mode. Sleep deprivation is displayed as an icon just below the AP bar. If sleep deprivation is left untreated it will directly impact available action points (AP) and SPECIAL stats.

Stages of sleep deprivation
Each of the stages below lasts for exactly 14 in-game hours, or 42 real-time minutes, before progressing to the next.