About
Tumblr After Dark refers a series of unitelligent or absurd posts made by Tumblr users during hours they should not actually be scrolling the internet and should be sleeping. Due to their lack of sleep at the time of posting, they are not functioning normally, resulting in posts and quotes that they would normally not post. These posts are often met by replies telling those users they should go to bed.
Background
Presumably (As the internet is open 24 hours) the sleepy bloggers or “Nightbloggers” are from the US. So late or night refers to mainly North America time zones.
Spread
Some users compile these posts in to longer ones using screen caps like so:
The posts tend to be quite long and contain many different screen grabs all from different sources.
More recently though a dedicated Tumblr after Dark account has been activated: http://t-afterdark.tumblr.com/