I wonder what is considered to be ‘adequate sleep’ then? For 20 years when I was driving trucks I only had around five or six hours sleep a night, even when I wasn’t working, and even now I’m long retired I rarely go to sleep much before 12.30am and am always awake by 6.30 and get up at 7am. If I stay in bed too long it makes me feel lethargic all day. Some folk need very little sleep and if this was just a ‘one off’ for the minibus driver I can’t really see that is an issue, though no doubt the court will think otherwise? 
Makes no odds to him now though.
Pete.