Mister Sandman

Mister Sandman is a Fallout 3, Fallout: New Vegas and Fallout 4 perk.

Effects
With the Mister Sandman perk, when a character is sneaking, the option to silently kill any human or ghoul while they're sleeping is available. XP from killing the target is earned. Mister Sandman kills give a flat 50 XP per kill.

Mister Sandman is performed in sneak mode and by choosing "pickpocket" on an unfortunate sleeping victim. The game pauses and brings up a menu with the options to "murder" or "pickpocket". Choose the former to activate the perk. When used, a short murder animation follows with the character cautiously looking around and slitting the victim's throat, accompanied by a cartoony jingle.

Possession of a knife or other bladed weapon is not necessary to perform a Mister Sandman kill, even though the kill animation shows the deed performed with a combat knife.

Fallout 4

 * Damage bonus works only on weapons with silencer mod installed, making it generally useless for weapons without that option, like energy or melee weapons.
 * The bonus damage stacks multiplicatively with the Ninja perk. With both perks fully levelled you get a 5.25x sneak attack multiplier.
 * Note, that while the multiplier displayed is rounded (e.g. 5.25 -> 5.3) the effective multiplier is not.
 * There appears to be a bug when combining any level of Mister Sandman with level 2 of the Ninja perk. The resulting mutipliers are 3.0, 3.3, 3.6 and 4.0 for Sandman 0, 1, 2 and 3 respectively. It appears the Sandman perk is only applied to the additional damage, so 1 + 2*SandmanModifier instead of 3*Sandmanmodifier.
 * It is not possible to perform Mister Sandman kills while wearing power armor.
 * As of patch 1.4, all three perks that increase Sneak Attack damage max out at times 4.8. Also it's worth noting that without this perk but with the other two (Ninja and Cloak & Dagger) the multiplier is 4.3.

Behind the scenes

 * The "Sandman" is a European folklore character who brings sleep and good dreams to people by sprinkling magic sand in their eyes.
 * "Mister Sandman" was a song recorded by the Chordettes in the 1950s.

Bugs
The 1.4 patch lowered the spread and max of multipliers: Ninja 3, Cloak, Sandman 3 = 4.8x Cloak, Sandman 3, Ninja 3 = 4.8x Cloak, Ninja 3, Sandman 3 = 4.8x Ninja 3, Sandman 3, Cloak = 4.7x Sandman 3, Cloak, Ninja 3 = 4.7x Sandman 3, Ninja 3, Cloak = 4.4x (!)
 * It is still possible to perform Mr. Sandman kills on Kenny from the Point Lookout add-on and Sally from the Mothership Zeta add-on after patch 1.5.
 * It is possible that while murdering the victim, they will wake, stand up and go hostile, however, they will still die after the murdering animation is complete. It seems that when the character being killed starts attacking the murderer, no damage will actually be dealt by them.
 * In Fallout: New Vegas ordinary characters killed by this perk may fall through the floor making it difficult to loot them before they fall.
 * For unknown reasons, in some cases, the Mister Sandman animation will be replaced with with the going-to-bed animation. As soon as the player character begins to lie in the bed, the NPC will begin to get out of the bed simultaneously. Upon lying down fully, the NPC will die and the PC will immediately return to the previous crouching position.
 * The bonus to the actual sneak attack damage multiplier on silenced weapons provided by this perk is known to behave erratically when interacting with Ninja and Cloak & Dagger. There does seem to be a bug with the calculations with all three perks active, depending on the order in which they were granted, as follows:

Pre-1.4 patch: Ninja 3, Cloak, Sandman 3 = 6.3x Cloak, Sandman 3, Ninja 3 = 6.3x Sandman 3, Cloak, Ninja 3 = 6.3x Ninja 3, Sandman 3, Cloak = 6.3x Sandman 3, Ninja 1, Cloak, Ninja 3 = 5.9x Sandman 3, Ninja 2, Cloak, Ninja 3 = 5.9x Sandman 3, Ninja 3, Cloak = 5.9x Cloak, Ninja 3, Sandman 3 = 5.4x (!)

Further testing needed, but right now the only solution appears to be selecting/receiving these perks in the "correct" order.
 * Sometimes the XP for kills via this perk will not be awarded.