It drains on its own, faster with creatures nearby, when you're alone or in an unstabilized chunk. Only Almond Water restores it.
Testimony
“Alone, in an unstable chunk, with something close by, that's how you lose your head. And nothing even has to happen, you know.”
Sanity drops with time and with circumstances. It is the number that separates you from losing your way. Nobody sees it and everybody notices when it runs out.
What drains it
Every second you spend down there you lose a little, and four things speed it up.
Source
You lose per second
Base: just being there
0.1
Per creature nearby
+0.3
Unstabilized chunk
+0.2
Being alone
+0.3
Cap
2.0
A wanderer alone in an unstable chunk, with two creatures nearby, loses 1.2 per second. With one more, 1.5. The figure climbs fast toward the cap.
How it’s restored
Almond Water. Restores 35 and is the only restorer. See the entry.
Waking up. If you die, you wake in a bed and sanity goes back to 50.
There is no other remedy. You can only lose it more slowly, and a stabilized chunk drains it less.
How to survive
Go with company. Being alone adds 0.3 per second, the same as a creature nearby.
Stabilize the chunk where you plan to stay.
Don't stand staring at anything that moves. Every creature nearby counts.
Save one Almond Water for the worst moment, not the first.
What people report
It is said darkness drains even more, up to 0.5 per second. The archive hasn't been able to measure it.
Below 50 you see things: objects that change place when you look back. Below 20, people talk about a pulse that trembles and aim that fails.
Some swear your breathing shortens and your hands shake before you lose your head completely. Others say notes get harder to read, the lettering tighter and repeating.
Some groups talk about shelters as points of calm.