It is quite common for airspace to be closed at night due to sickness. In theory the closure is notam'd, although I know of several occassions where this was not done. On each occassion this was to cover up failings in the process to arrange cover. I understand it is a contravention of the licence if nats can't provide the service which it is supposed to, the odd time is no doubt understandable but it is becoming much more commonplace. It is possible to keep closures like this one hidden from everyone outside the atc operation including srg and the caa, which is why sometimes it isn't notam'd.