To put it clearly: it starts at 4:30 am, Tokyo time.
People in Japan live in a different time zone as people in the UK. When the night starts in the UK, Japan reaches the very last hours of the night.
This is due to the Earth's endless rotation on itself: when some parts of the Earth's surface are exposed to the sun (and are therefore in day light), some other parts are in the night; and because our planet rotates on itself, at some point, parts which were in the night become lit by the sun, and parts which were under the day light start entering into night. And the cycle repeats endlessly.