Actually, it varies. Two genetic tests an experience confirmed I have much slower caffeine metabolism. So no coffee after 11 for me, and ideally not a lot.

I just fact checked myself and found that the half-life is actually 3-5 hours. Sorry about that. But in any case, it's common for people to go to bed at night with some active caffeine in their system.

Does this mean that after 8 hours the caffeine is still in my system, acting half as strongly as it was at the time of ingestion?

Well, there's half as much, I don't know enough to say there is a linear relationship between caffeine concentration and it's psychological effects.