Minecraft wiki entry for Crawling, associated text: In these circumstances, the player automatically switches to a crawling state. When enough space opens up above the player's head, the player automatically exits this state and begins standing again. When crawling, the player travels at sneaking speed. In the crawling form, the player is reduced to 0.625 blocks tall, or 5⁄8 of a block, meaning that they can crawl over blocks 0.375 blocks tall, or 3⁄8 blocks tall, the height of a daylight sensor.