In the morning a wizard prepared all his spells. He did not used any of them on that day. He went to be an woke up in the middle of an encounter. He used one (1) spell. The encounter and following actions took 1 hour.
The wizard go back to his bed and rest a couple more hours. Can he replace the used spell or 8 hours of sleep are required even if only 1 spell was used?
i believe RAW states you need 8 hours after casting a spell slot to replace that slot. so yes you need to rest.
The 8 hours sleep is necessary to regain that one spell. However, every other spell that he had memorized (but not cast) will still be ready for hm the next day.