I checked 4 times with my supplier. When I went over to them in '99 I was told 12.30 - 7.30 (summer time), checked again in April 2006 as I felt my night usage was not high enough - answer given by email was the same, still not happy with that and by pure luck looked at my meter at 22.35 and the night time indicator was on. After emailing and phoning I have been told that it is from 22.30 - 0.00, 0.00 - 0.30, 2.30 - 7.30 (still confused) however I have monitored this myself (getting up during the night) and it's 22.30 - 0.30 - 2.30 - 7.30. Now arguing with the supplier as I've been putting on my appliances on at 0.00 for up to 2 hours, therefore using up to 1.5 hours of daytime units since I don't know when!!!!!