Wow, THAT was odd! A noise woke me up - workmen operating machinery outside. I felt totally exhausted and had a headache... 
Then my alarm went off, at the correct volume this time. Obviously I couldn't have adjusted it properly the morning before. The lack of sleep is not doing me any favours! 
I felt too groggy to get up so I decided to go back to sleep. I closed my eyes but was immediately startled by another loud noise outside. 
Somehow or other the fright seemed to have cleared my groggy head and fatigue. And then I noticed the time - in what felt like just 10 seconds I had actually slept for 70 more minutes! 
 
My body is telling me to get more sleep at night. As soon as I relax during the day I doze off. It takes me about 90 minutes to watch my evening recording of the 55 minute Ch4 news because I fall asleep and wind back the bits I missed! 
Hmm... Warfarin, but that would be 6 hours or so. That is worth looking into. Thanks.