Topic particle after "expression of time"


I am reading chapter 4 from the JLPT N5 book, where I found the following
sentence:

> 昨日勉強しましたか。

After 昨日, why wasn't there a topic particle? Is "昨日は勉強しましたか。" an incorrect
sentence?




Marking it with は is certainly not wrong, and would in fact be natural. It
doesn't change the meaning, but if the は is stressed, it will contrast
yesterday to all other days, and will thus put stress on the time:

> 昨日勉強しましたか - Did you study yesterday? (Plain question)  
>  昨日 **は** 勉強しましたか - Did you study **yesterday**? (As opposed to any other
> day)

However, if the は is not stressed, then は is a normal topic marker and it is
still a plain question:

> 昨日は勉強しましたか - Did you study yesterday? (Plain question, は unstressed)

(Thanks to naruto for pointing this out in the comments.)

