Reducing sleep duration for work or studying for tests is one thing, but I did not imagine someone would wake up in the middle of their sleeps to accomplish their goals. My personal experience says reducing sleep duration make me less functional.
Review time is based on the SRS stages. It will show up after a certain hours has passed. If WaniKani wants to prevent the reviews to show up at the time where people is supposed to sleep, it would be a challenging problem to solve. First, people have different schedules. There are people with night shifts. Second, WaniKani users are everywhere in the world. We live in different time zones. Perhaps, WaniKani can solve this by adding user time zone and user time preferences. If WaniKani does implement them, the next question would be SRS and memory theory. If both SRS and sleep are proven to help memory retention, what if sleep makes one passes SRS time?
Speed running WaniKani is a side effect of the gamification. WaniKani gamifies by having levels and a final level. There is a clear and attainable goal and we often satisfied by progressing. It can be addicting for some people. If WaniKani wants to have a health warning when a review happens at night, it needs users’ time zone. One of the easier ways is by putting a warning every time users open WaniKani.
The discussion we have here is already a health warning in itself. A reminder to have self-control. 
