Why do so many medications cause drowsiness?

Question

I'm just wondering why this is such a common side effect.

Answer 1

I guess sometimes, it's because those drugs cross the blood brain barrier?

Because from what i've read in some articles, certain drugs that don't cause drowsiness are those that do not cross the blood brain barrier.

I might be wrong though.

Answer 2

Somnolence (or "drowsiness", or "hypersomnia") is a state of near-sleep, a strong desire for sleep, or sleeping unusually long periods. It has two distinct meanings, referring both to the usual state preceding falling asleep, and the chronic condition referring to being in that state independent of a circadian rhythm. The disorder characterised by the latter condition is most commonly associated with users of prescription hypnotics, such as Mirtazapine or Zolpidem.

It is considered a lesser impairment of consciousness than stupor or coma.