I try to avoid looking at my bank balance - I don't enjoy panic attacks! (Unless there's a particular reason to check it, I only look at it once per month when my printed statement arrives).
However, from the days when I've had to check on it carefully (because I've only had 3p to my name on one occasion when I was unemployed, counting both what was in my bank and in my pocket), I know that 'ordinary' direct debits INTO my account would normally be there at one minute past midnight. However I've had 'express' direct debits paid into my account (e.g. when a cheque that I've had, from a firm I've worked for, has bounced, and I've then complained) within minutes of the firm telling me that they've paid the payment (so that could be at any time of day).
Direct debits going OUT of my account also seem to have been taken from my balance at one minute past midnight. As far as I can tell, card payments are also processed on a daily basis in many (all?) cases, rather than the money actually disappearing from my balance as soon as I buy a pint in a bar.