Hibernation

Hibernation forms part of the core gameplay loop of Rain World, serving as the goal of each cycle. Shelters, dens or hibernation pods are self-sealing chambers scattered around regions, used for hibernating: resting in them is the only way to finish a cycle, save the game and progress karma level. After hibernating, the save's cycle count as well as cycle timers throughout the world advances.

Function
Hibernation is initiated by entering a shelter room with sufficient food to sleep through the rain. The total amount of food required to sleep at shelter is indicated by the food meter at the bottom left corner of the screen, which can be viewed at any time by tapping or holding the map button.

The food necessary to hibernate varies between difficulty; the dividing line indicates how many pips are required to sleep, with any pips beyond it being stored for the next cycle:
 * Monk :
 * Survivor:
 * Hunter :

Hibernation can be initiated at any point during a cycle, but must be done before the rain kills at latest. Entering the chamber of a shelter and staying still whilst possessing the minimum amount of food initiates the sealing process and brings up the hibernation screen. As long as movement inputs are being performed, the shelter doors will not begin to close. If lacking the minimum food whilst inside a shelter, any unfilled food pips below the line flashes red, unless there is enough food for hibernation present in the shelter in which case it is automatically consumed and hibernation is initiated. Similarly, any s inside shelters are automatically consumed when hibernating. If entering or inside a shelter whilst it is raining, hibernation is forced as soon as Slugcat is inside the chamber.

Shelter interiors are not affected by the rain, and it's possible to stay indefinitely inside the tunnel into the main shelter chamber without being killed or forced to hibernate.

Once at the hibernation screen, the amount of food consumed during the cycle is shown, as well as current karma level and passages if any are in progress or attained. It is also possible to view the map by holding the map button. Holding the jump button will speed up menu animations.

Starvation
Hibernation can be forced without sufficient food as long as at least one pip is full. Starvation is initiated by holding the down button in a shelter, during which the dividing line begins to shake and travel leftwards to meet current pip count. Letting go of the down button before the line meets the final pip cancels the starvation attempt. Alternatively, entering a shelter whilst it is raining and without sufficient food automatically forces starvation. Upon successfully initiating starvation, all pips, as well as karma reinforcement if present, are consumed and the hibernation screen is brought up with a warning that progress has not been saved.

Starvation effectively acts as a double or nothing bet: it is a useful tool that grants the chance to gain further progress without sufficient food at the expense of severe drawbacks and the risk of losing it all when dying. The cycle after initiating starvation is referred to as a "starve cycle".

Consequences of starvation include the game not saving properly, Slugcat requiring a completely full food bar in order to hibernate again, as well as receiving significant debuffs. Their throwing skill is reduced to Monk's, lowering damage and limiting slide-throwing. They intermittently enter a lethargic state where, if exerted too much (notably by jumping excessively), they close their eyes, moving slower and having weaker jumps whilst they attempt to regain composure - similar to exiting water with very little oxygen left. When completely exhausted, they cease function and drop any items currently being held. They also become a paler, more desaturated tone, visibly skinnier, and weigh less than on a normal cycle. Completely filling the food bar removes the debuff and allows for successful hibernation.

Dying or quitting out when starving results in being rolled back to the last successful save, meaning physical progress across the map and reputation changes made during the cycle before and after forcing hibernation is discarded. However, map progress is still retained. Attempting to starve a second time after starving also simply results in death, presenting a unique death screen.

Quitting on the hibernation screen when starving does allow for continuing from that point when resuming the game, but quitting out in-game after this still counts as a death even when within the 30 second quit-out threshold.

Bugs & exploits
Due to starvation's unusual interactions with how the game saves progress, there are several bugs (issues with the game's code) and exploits (unintended usage of intended mechanics) associated with it.

Passage progress gained on starves, including completing a passage, is retained even if the starve cycle is not successful (not accounting for certain passages losing progress when having died). is the only exception to this exploit as reputation changes are rolled back after dying.

Karma caching is an exploit that allows for quickly gaining karma without having to survive or advance cycles. It is executed by starving, entering the next cycle and then quitting out before the 30 second quit-out threshold. This retains the karma increase whilst reverting to the cycle before the starve due to the game not saving, and can be chained multiple times in a row to quickly raise karma as much as needed. This exploit can also be used to obtain, (provided there is a tamed lizard present in the shelter) and  passages. This is predominantly used in speedruns.

Double-spawning, whilst not exclusively triggered by starvation, is a bug that doubles all den spawns in a region if the cycle in which it is loaded for the first time in a playthrough ends in a starve. Due to how starving does not save progress, the game loads the list of spawns again as it believes the region has been entered for the first time again. Dying after starving reverts the doubled spawns, and the bug can no longer be triggered for that region. This is predominantly used in scoreruns.

Starving into  is an exploit that involves starving and visiting an Echo, causing progress to be reverted back to the cycle before starving whilst still maintaining the karma increase (along with passage progress made the cycle before visiting the Echo). This is predominantly used in runs to conserve cycles, or to warp back to specific shelters.

 starve flight is an exploit caused by an unusual interaction between reduced weight when starving, jumping states and the effect white Squidcadas have on Slugcat's gravity. The most common method utilised to trigger it is backflipping whilst holding the Squidcada, after which height can be gained infinitely by holding the jump button. Starve flight can be used to clear long distances while staying in the air, making it useful for crossing gaps that cannot be cleared by typical jumps, or the large patches of that can be found in Farm Arrays. Flight can be stopped by grabbing a pole or letting go of the Squidcada.

It should be noted that with the upcoming parity patch these are all subject to change.

Storage & creature interaction
Objects (other than Karma Flowers), food, creatures (other than ) and corpses located inside shelters can be stored indefinitely as long as they either cannot get out on their own or are not taken out by s. They persist no matter how many cycles pass and when the region is no longer loaded. The map displays icons (up to a maximum of 50) underneath the shelter to represent what is stored inside it.

Most objects (with the exceptions of s,, hydrated , s, Hunter's green neuron, , s, 's neurons and s) do not have collision with Slugcat and all objects cannot go through pipe connections without being held by a creature, hence theoretically an unlimited amount of objects can be stored in shelters. However, this may cause performance issues.

Creatures are capable of entering shelters and may pursue prey into one if they witness them entering it. It is also possible for creatures to enter even when the shelter doors are closing, resulting in them being considered inside the shelter and being placed inside the main chamber when the next cycle begins. As a soft-lock prevention measure, any creatures present in a shelter at the beginning of a cycle are pacified for 15 seconds as long as they are not aggravated. However, this does not appear to work with larger, meaning care should be taken to lure them away from shelter entrances and/or kill them if planning to hibernate when one is present. Occasionally, hibernation can be inadvertently triggered if a creature (such as ) brings an incapacitated Slugcat into a shelter chamber whilst they meet the food requirement.

s, s and all except s and s die if inside a shelter when the cycle ends. also loses its attractive quality the longer it is stored.

When brought into a shelter and hibernated with, Noodlefly Eggs hatch into several infant Noodleflies, which follow Slugcat until an adult Noodlefly is encountered, at which point they begin following the adult.

It should be noted that it is possible for objects, creatures and corpses to clip out of shelter geometry, in which case they cannot be recovered unless the cycle is restarted via quitting out.

Passages
See main article: Achievements

Passages can be viewed and used from the hibernation screen. Passage progress is displayed in bars extending radially from the karma symbol on the hibernation screen, which glow a bright white when completed. Passages with pips have requirements that can be completed non-consecutively, whereas passages with bars must be contributed to consecutively until the passage is attained. The only exception is Chieftain, which instead is tied to global reputation and can be revoked if enough Scavenger reputation is lost.

Selecting the passage button consumes the oldest attained passage, displaying the passage art before prompting the player to pick a region to travel to. Buttons at the sides allow for scrolling through each region that has been visited previously and the map button can be held to pick a specific shelter to spawn in (being left random if not selected). Leaving the passage screen at any point before confirming cancels its usage and brings up the main menu screen.

Passage fast travel is not available on Hunter due to being repurposed as score multipliers.

Dreams
Whilst progressing through the game, special one-time cutscenes occasionally play after hibernating when certain conditions have been met. They are indicated by the screen fading to black before getting to view current karma and passage progress, and can be viewed for any duration until the continue button is selected.

Dreams can be separated into 4 categories, in descending order of priority: Iterator, Void, Overseer and Family dreams. All of them can be predicted thanks to their defined requirements.

Iterator dreams happen after encountering Looks to the Moon or for the first time. Depending on actions taken during the first visit, there are two variants of the dream that can play after visiting Moon.

Trivia

 * It is possible that shelters are not a real aspect of Rain World's setting, but instead a stand-in for rain-proof areas such as air pockets, since shelters are easier to implement from a programming and design perspective.
 * At release, starving at any point simply resulted in death (using the same death screen currently used for starving twice in a row). This was changed to the current starvation mechanic with 1.5.
 * In alpha versions, shelters were considerably different visually, exhibiting two airlocks on the sides with two pipe connections outside each one. They can be seen in the Slugpups video.