My phone keeps turning itself off anyway, even when the battery's fully-charged and I don't want it to!
A problem with texts though is that they occasionally seem to get 'stuck' in the system. A friend recently sent me a text at about 7pm but I didn't get it until just after midnight. In the past I've received texts up to two days after people have sent them. So Sainsbury's might have actually sent that text far earlier than when you received it. (If you're on their lists for receiving texts, simply texting back 'STOP' will probably get you taken off).
When I used to work night shifts though I tried to remember to turn my mobile phone off before I went to bed. These days I'm also often nocturnal by choice, so the ringer on my home phone is always turned off. (If I'm sitting at my desk, I'll see the light flashing; otherwise the call will go unanswered). My mobile is theoretically left on but, as I wrote above, it will probably have turned itself off anyway!