I know from Memory Alpha that sleeper ships were used as late as 2210, when a vessel of this type was sent on a deep space exploration mission to Beta Capricus. ("11:59") So, it's possible that the Valiant was a sleeper ship. It's not possible to know for certain if the Valiant had a crewman who was awake to monitor the ship and its crews, like the deep space explorer mentioned previously, or that the leader was sleeping like the rest of his crew and the ship's computer automatically monitored the ship and its crew, ex. Botany Bay.
I suspect a non TOS source here again with more erroneous retconning.