Translation:Yesterday I went to bed earlier than usual.
I believe the phrase you're thinking of would be either "usually" or "like usual." But those would be used when communicating that something is THE SAME AS the usual thing. The sentence in this exercise is making a contrast, communicating that the thing is DIFFERENT FROM the usual thing. So: "later than usual," "bigger than usual," etc.
Why not? Чем is a conjunction required in English, but not required in Russian
Я быстрее, чем он (Я быстрее его). I faster than him
Это лучше, чем то (Это лучше того) It is better than that.
Книга интересней, чем фильм (Книга интереснее фильма) The book is more interesting than the movie.
Russian uses a comma in comparisons and with a variety of other scenarios. For example;
Он лазит по деревьям, как обезьяна. He scrambles about in the trees like a monkey
Кто-то научил своего малыша плавать раньше, чем тот стал ходить. Someone has taught his baby to swim before he could walk.
These examples are from Terence Wade's "A Comprehensive Russian Grammar", it is a very useful book alongside Duolingo.