A colonist's mood is the sum total of the effects of their thoughts and traits. If a colonist's mood falls below their mental break threshold for a period of time, they may have a mental breakdown - with severities and 'frequency' depending on how far below their threshold they are.
Colonists have a base mood of 50%, and their mood threshold changes according to difficulties. Each difficulty has the following impacts on colonist mood:
With the offsets; it's reasonably easy to keep people from breaking on the Rough and Intense difficulties, but it's much more difficult to keep colonists from breaking on Extreme (as the baseline mood offset for Extreme is effectively Rough, but with colonists constantly having the 'Feeling Terrible' thought)
With a sufficiently high mood, Colonists will be more inclined to work harder if they are in high spirits; with global work speed starting to improve when they are 'Content' (mood at or above 60%), with as much as a 20% increase in global work speed if their mood is at 100%. As of Alpha 15, colonists retain their full work speed if in poor moods.
As of Alpha 15 (August 28st, 2016), mood is subtly drawn behind the pawn on the colonist bar at the top of the screen.
A mental breakdown occurs when a colonist's negative thoughts outweigh their positive thoughts enough to push their overall mood below their mental break threshold. As aforementioned: a colonist will experience a mental breakdown if their mood is at or below their mental break threshold - and the severity of a mental break depends on how far below their threshold they are. Once a colonist composes themselves and brings themselves out of their broken frame of mind; they'll get the 'catharsis' thought, granting a +25 mood for the next 2 days. This will generally prevent a colonist from entering a spiral of mental breakdowns in all but the worst of the worst scenarios.
While a colonist is experiencing a mental break, you have no direct physical control over them - but you can draft another colonist, right click on the broken colonist, and attempt to arrest said colonist. The broken colonist will either go berserk, or they'll come quietly and be escorted to the nearest available prison bed - immediately ending their mental breakdown afterwards. You can then open their 'Prisoner' tab and set them for release, and then anybody assigned to wardening will 'release' the broken colonist back into your colony. However: this eliminates the catharsis bonus, and instead yields a 'was imprisoned' thought.
The following is a list of all possible mental breakdowns a colonist may experience at appropriate severities as of Alpha 15:
Like the dazed and wandering event, only shorter. The only difference is that those in a Sad Wander is that, as opposed to a daze; they won't strip off clothing, weapons, or inventory. Confused wandering can also be triggered if a pawn has Alzheimer's or Dementia, even if they do not have a poor mood.
Hide in room
Colonists who hide in their room will wander about in their room, refusing to come out. This event is relatively short and - barring insect hives - does not put the colonist in harms way.
Colonists may binge on either food or social drugs (i.e. smokeleaf and beer) if available, in a bid to distract their minds from all the hardships that are getting to them. During this mental state, the affected colonist will frequently consume whatever they choose to binge on, and generally wander around in brief intervals. This can be a minor concern because the affected colonist will frequently consume precious food (particularly early game) in the former case, or they may develop an addiction to one of the aforementioned drugs (alcohol withdrawal is particularly painful to get over).
A binge will end as soon as the affected colonist gets back to bed, or after a certain amount of time has passed. Social drug binges will also occasionally happen for those with the chemical interest trait regardless of mood, and still gain the catharsis thought afterwards.
Dazed and Wandering
Colonists who become dazed will wander around and will eventually recover on their own. One that's in a daze will gradually strip off their clothing, weapons and inventory. Dazed colonists can also be arrested, which can be useful if they are in imminent danger. Your colonist must be in combat stance (hotkey R) to bring a dazed colonist to a cell (just like a prisoner). Once imprisoned they may immediately be set for release. When released by a warden they will revert from a prisoner back to a colonist, with a mood debuff. Dazed colonists may resist arrest, at which point they will go berserk.
Colonists on a hard binge will binge on hard drugs (i.e. psychite (flake and yayo), go-juice and wake-up). Hard drug binges are considerably more dangerous for the affected colonist's health than soft drug binges, as hard drugs contribute towards overdoses which are potentially lethal. As with soft binges: hard binges will either end when the affected colonist goes back to bed, or after a certain amount of time has passed. Colonists taking malari-block and luciferium have nothing to do with the drug binge they are on.
Colonists with the chemical fascination trait will occasionally have a hard drug binge regardless of mood, and still gain the catharsis mood afterwards (if they don't die from an overdose).
When a colonist goes berserk, they become a danger to themselves and anyone nearby. They will melee attack the nearest living creature until their target is either subdued or killed. Due to the risk of death or permanent injury for both the psychotic colonist and any that happen to be nearby, a psychotic colonist is a very real threat. If the berserk colonist poses a direct danger or is in direct danger, one or more bare-fisted colonists work best to quickly subdue them with the least risk of killing them. Once a berserk colonist is subdued they do not need to be arrested, rather they can and should be rescued to recover from their injuries. If the berserker is not in danger and can be safely avoided, it's best to let them rage until their berserk rage wears off to avoid injuries and the associated pain mood debuff.
Colonists with the pyromaniac trait can go on a fire-starting spree. They will attempt to ignite any flammable material. Do note that those with the Pyromaniac trait will go on fire-starting mental breaks anyway, regardless of mood, and get the Catharsis thought afterwards - even if well above their minor break threshold.
Animals may suffer a manhunter mental break, similar to berserk. A manhunter animal will seek out and attack any humans on the map it can reach. The mental break can be triggered by the death of their bonded master, a psychic wave or by being injured by a colonist, such as when hunted. A player should take care about what animals they hunt, because although most species rarely turn manhunter, others have no tolerance. If one animal in a herd is angered, the entire herd may turn manhunter. Since alpha 14, animals will break down structures and doors, so colonists are not safe behind them. As with other mental breaks, given enough time, it wears off.
The following images document our Volunteer and 'Break Savant' Stingray's decline of sanity as she undergoes a journey not of the physical kind, but of mentality:
Mood is a Stat: How happy or stressed someone is.