(covers information from several alternate timelines)
This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
A night terror was a type of sleeping disorder.
In the mirror universe, as a child, Michael Burnham suffered from night terrors. Although she spoke little during the day, at night, Burnham would scream and fight to escape in her sleep. Her adoptive mother, the Emperor Philippa Georgiou, would hold her down as she struggled. One night, Georgiou did not hold her daughter down and waited to see what would happen. She witnessed her daughter sleepwalking to a field of fireflies. Once her daughter became calm, Georgiou carried her back to her bed. For every night, until Burnham no longer suffered from the night terrors, Georgiou would walk with her to that field. The next day, Burnham did not remember what had transpired over the night. When Burnham outgrew the night terrors, Georgiou felt sadness.
In an alternate timeline experienced by Georgiou via the Guardian of Forever, she visited her daughter in the brig aboard the ISS Discovery. The emperor reminisced about this period in her daughter's childhood. When she thought about it in hindsight, she no longer felt confident that Burnham had outgrown the night terrors. Unveiling a transparent bowl filled with fireflies, Georgiou gently placed it down near her sleeping daughter's head. (DIS: "Terra Firma, Part 2")