The Nikkei Index is watched closely, especially at the moment on speculation around the state and future of the Japanese Economy.
If you check over Yen pairs (Forex) you will notice the trajectory looks very similar inline with Yen weakness, that the BOJ / Govt seems happy to see continue.
This, ultimately is reflected in the index. Any real turn of Market Sentiment may push us significantly to the downside, or indeed to upside in a risk on scenario. Awaiting news events to finish the week.