How does Cas know Dean is an angry sleeper?
Because Cas has watched him sleep many times, and so many of those times, Dean has been a wee bit grumpy when Cas woke him up; like that time he was sitting on Dean’s bed in 4.03 and Dean was like WTF:
Which was right after this angry Dean after being woken up in 4.02
I mean it wasn’t really about being woken up that time but it was still a mark in the wow Dean’s a bear if you wake him up before he’s ready column. Another one for that column when they had this confrontation in the middle of the night in 6.20:
And then there’s this in 8.10:
And it’s implied this has happened many times before:
And it makes sense because we know Cas and Dean have shared a motel room on cases (8.07):
And that Cas sleeps at the bunker (12.02):
Where we can infer he’s seen Grumpy Bear go into hibernation with some coffee. And then there’s this Before He Even Goes To Sleep Grumpy Bear when Cas wants to pop over and Dean’s like no way dude I gotta sleep I’m so freaking tired (5.04)
And when Cas agrees, Dean just grumbles some more and hangs up
Cas has definitely witnessed Grumpy Bear on many occasions, and that’s how he knows.