Summary: After years of abuse and neglect, things take a dark turn for Sans. (Underfell)

prologue

When a monster is close to 'falling down' they go through a series of different stages depending on the monster's HP, but it all comes down to eight main stages. First, they have fierce coughing fits, and can be mistaken for the flu.

The next stage is the loss or near loss of voice, which can be dismissed easily by those who don't check the inflicted monster's HP.

The third stage is the decrease in magic/magical abilities, which may affect the inflicted monster's physical appearance and mental state.

The fourth stage is the loss or near loss of eyesight, and can cause the sequence of falling down or self destruction to happen quicker or easier.

The fifth stage is the isolation of one's self, and nightmares from previous memories. These two happen simultaneously, in addition with stages 1 through 4. Some call the nightmare bit '5.5' since it happens overnight instead of 24 hours. During these nightmares, the inflicted monster will not be able to wake up, until someone else wakes them up instead.

The sixth stage is the loss or near loss of hearing, which may cause the victim to self destruct rather than fall down. Also, it is harder to wake up the victim from a nightmare, and can cause a comatose, which would slightly increase the victim's life, but this does not stop the inevitable.

The seventh stage is the sense of getting affairs together and extreme hopelessness. The victim would complete tasks that they haven't done before the sequence of falling down, although some call this part not a stage, but acceptance.

The eight is finally falling down.

From the Journal of Dr. Alphys

Information from Project Determination