Free usage query

[#1]

I am new to this and apologise in advance for my lack of knowledge.
When my paid for usage has been used up it seems to stop the free usage after midnight - is this correct and if not how do I ensure I am still able to use the free usage after midnight?
Thanks for your help

[#2]

Are you on PAYG or Broadband Your Way?
If you are on Broadband Your Way, then when you hit your monthly usage allowance you can either add more - or have all your traffic restricted to 128kbps.  This includes the overnight usage.
If you're on PAYG and choose not to add extra usage (and you hit your usage allowance) then data transfer stops - and this includes overnight usage too.
This is by design - so to ensure you get decent overnight speeds and free overnight usage - stay within your paid-for allowance (or add extra).