Full Moon

In Chapter 6 we learn that the Full Moon (満月, Mangetsu) has a powerful effect on Monsters, and according to Centorea it is "A night that calls out their instincts with the utmost force." Thus making their passions and behavior more extreme and reducing their self-control.

During the chapter, this combined with the possibility of Kurusu being able to marry one of them caused the girls to lose their control and use their strength in full, to the point where he feared for his life.

Amusingly enough, the way to apparently reduce the effects is to literally cool their heads. The morning after, it was shown that the girls have little to no recollection of their behavior while under the effects of the moon.

After learning of this firsthand, Kurusu determined that his home stays must go to bed early on nights with a full moon, and has apparently not had another incident since.

Trivia

 * This is an obvious allusion as to how animals and people are supposedly affected by the full moon, bringing out their more bestial and primitive nature.