Sleep is a measurement of the raw time you've spent awake: the longer you are awake, the higher your Sleep value is.
Beverages containing a bit of caffeine will reduce this value, as will stopping your adventure and taking a nap. Furthermore, sleeping in a bed you own or have rented will get you the Well Rested bonus for 12 hours.
As your sleep deprivation worsens, your Agility and other stats will be affected. Not sleeping for two weeks will cause you to die.
Although there are some Aids items help with reduce SLP deprivation, No-Sleep gameplay may not be possible unless you are prepared to skip a few quest require Sleeping. The effect of no-sleep on Mojave is uncertain, although on Zion Honest Hearts it seems to speed refresh rate up.
pcps3xbox360 Sometimes the SLP meter in your 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 your character will not advance in sleep deprivation. This bug may be fixed by finding a bed and sleeping for any amount of time. After the player wakes, the gauge will begin displaying a number again and will increase over time normally.
pc In PC v140525, starting Honest Hearts by travel to Zion will stuck the SLP meter at non-number. As long as Courier is not sleeping at any location, it will not change. sleep once and the bug will be fixed. It has the effect of making the respawn rate in Zion's plant life, animal life, and merchants shop, operate much better than if you fix it. Barring future patch fixing it, you probably should not sleep in Zion. [verification needed]