These 100,000 years worth of sleep were recorded in the app between July 16th, when the app had its first limited launch in countries that previously had access to the open beta, and September 13th. Given over a month has already passed since then, the total time is no doubt significantly more by now. No reason was given for why they waited this long to announce hitting this milestone.
Sadly, at this time there has been no announcement of any in-game gift to celebrate this milestone, as was the case when the game announced hitting ten million downloads worldwide back in August.