Hostile mobs in Minecraft spawn only at light level 0, meaning complete darkness is required for their spawning on the Overworld surface and caves. If the light level is 1 or higher, hostile mobs will not spawn there

. To clarify:

• Light level 0: Hostile mobs can spawn.
• Light level 1 or above: Hostile mobs cannot spawn.
• Light levels below 7 (including 0) were historically relevant, but the current standard for hostile mobs is strictly light level 0.
• Passive and neutral mobs have different light level requirements, often needing light levels of 7 or higher to spawn

There are two types of light in Minecraft:

• Block light (from torches, lanterns, etc.) which prevents mob spawning at level 1 or higher.
• Sky light (from the sun and sky exposure) which prevents spawning if at level 8 or higher.

Nighttime itself does not create a light level of 4 on the surface; rather, surface blocks exposed to the sky at night have a sky light level of 4 but a block light level of 0, allowing mobs to spawn in dark patches like under trees or overhangs

. In summary, for hostile mobs to spawn, the block light level must be 0, i.e., complete darkness at the block where the mob would spawn