The requirement to staff sectors is for two people (one Tactical, one Planner) to be in position at all times. In order to provide legally-required 30-minute fatigue breaks every two hours during the night, there must be at least one spare person who has the same validation pairing. In this case, due to sickness, the third person only held a DTY validation and therefore when he was due to provide a break for the two combined CLN/DTY sector controllers (30 mins break each), the CLN sector had to be closed for one hour. Due to the difficulty of accurately controlling traffic flow to ensure there was nothing presenting for that hour, the decision was made to NOTAM CLN as closed from 2300 to 0530 and then allow traffic through when the dual valid controllers were in place. I don't know whether that last option was actually used or not or whether the traffic avoided CLN completely all night.