Sleep

Sleep or Insomnia is a derived statistic in Fallout: New Vegas and Fallout 4.

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

Fallout: New Vegas
In Fallout: New Vegas, sleep deprivation (SLP) is only measured in hardcore mode. 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.

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 travelling 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, insomnia is only measured in Survival mode. This illness, if left untreated, directly impacts available action points (AP) and SPECIAL stats.