Let's take a look at the previous four nuclear tests by the DPRK. What we see is that these tests are all carefully timed.

The DPRK conducted nuclear tests in 2006, 2009, 2013 and earlier this year. The fourth one marked what the country claimed to be its first hydrogen bomb. The tests were each followed by strongly-worded United Nations Security Council resolutions.

The tests coincide with important domestic anniversaries and international meetings. Analysts say the DPRK aims to enhance internal unity and gain leverage to force talks with the United States.