That heartbeat is your last warning to stop making noise ASAP. They will start jerking around awkwardly and you'll hear a loud heartbeat sound. They will only enter heartbeat mode if you continue to make them mad while they're glowing - taking one too many steps, for instance. They also start making the distinctive 'clicking' sound you've heard them make, which is a quick to tell if a room is empty or not - don't hear any clicking? It's probably clear. They also, well, start glowing, which can be useful in some particularly dark rooms. When they're glowing, their detection range is increased, letting them hear things from further away (importantly, including 'killing other sleepers in the same room'!). Once they fall back asleep, you'll then get a few seconds before any of them start glowing again, letting you sneak past them more easily. The 'shining your flashlight on them' part is significant - you can 'sync up' a room by just sweeping across it with your flashlight, which should make everything in range start glowing. They can also be pushed into the glowing phase by shining your flashlight on them, or making non-gun noise (jumping, walking, hitting a lock with your hammer, etc). When they're asleep, they're totally silent, and will randomly enter the glowing phase.

The sleepers will have 4 'phases' - asleep, glowing, heartbeat, alert.

You want to bonk everything you can with that hammer. Do not sprint until you are certain a room is empty, or the sleepers are already attacking you. Do not shoot until the sleepers are already attacking you.

Gtfo tips and tricks 2021