Stone Sleep is the natural phenomenon in gargoyle biology that causes them to turn to "stone", or rather, an organic substance similar to stone, in the daytime. Despite appearances, it is not activated by sunlight coming into contact with gargoyles but by gargoyles' internal biological clocks, which are strongly tied into the Earth's bio-rhythms.

When gargoyles enter stone sleep, their whole body transforms into the same color and consistency. Stone sleep last from dawn to sunset, except in some odd cases as when gargoyles are being rapidly transported to different parts of the planet, causing their biological clocks to be somewhat off in a variant of jet lag. When a gargoyle awakens at night, he or she turns to flesh and blood from the inside out, finally shattering a thin layer of stone over himself or herself.

During their time asleep, gargoyles heal from their wounds, purify their bodies of toxins and recover from any illnesses that they might have; in most cases, they will awaken from stone sleep fully cured. There are occasional exceptions to this rule if the damage inflicted upon a gargoyle is severe enough. Extreme damage done during stone sleep may result in the death of the gargoyle, in which case they wouldn't wake up at all.

Gargoyles' clothing turns to stone with them. Originally, this was not the case; their garments (if any were worn) would remain unchanged, and consequently, would get torn apart when the gargoyle awakened. This came to an end in A.D. 10, during the reign of Caesar Augustus, who, displeased with seeing gargoyles rendered naked upon their awakening this way, had his adviser, the Roman Magus, cast a "Spell of Humility" over the entire race that caused their garments to become affected by stone sleep. This spell also causes any possession that the gargoyle is holding at the time and considers to be his or her own to, turn to stone as well.