What is a word that means before sleeping or before bed, like how postprandial means after dinner?
Just as post + prandial [of or relating to a meal (as dinner)] means "after dinner," so pre + somnial [of or relating to sleep or dreams] can mean "before sleep" or "before bedtime."
"But I was not yet able to trace my day-dreams back in the morning, as I did those of the pre-somnial state." [J. Varendonck, The psychology of day-dreams]
Bedtime, or is that too childish?
There is a very obscure Latin term used by medical professionals on prescriptions:
- Hora Somni (H.S)
At the hour of sleep; Medical shorthand for "at bedtime". (Source: Wikipedia..)
I think you're looking for "hypnagogic"