Tiredness = irritability - in action

Good day to you all.
For a while now I have been a lot calmer, especially with things that used to wind me up - annoying co-workers, bad drivers, incompetent customer services people on the phone etc.
But for the past couple of days, I have been feeling really tired when I wake up. I have had a good nights sleep, unbroken (for the most part) and I've been waking up at my normal time. But for some reason when the alarm goes off I feel like I've been awake all night.
Because of this I can feel / see / hear myself being more irritable and intolerant. This morning for example, driving to work, I got stuck behind a white van - literally, he pulled out in front of me as I was leaving the end of my access road. He drove 10 miles an hour below the limit all the time I was behind him, kept braking sharply because he couldn't judge the width of his van, and kept swerving out way too wide to avoid parked cars.
I know that a few days ago I would have had this sort of experience and not been fussed about it, I would just shake my head and laugh gently to myself when stuck behind some brain dead pile of monkey spit who couldn't drive. But because of this new tiredness, this morning I found myself turning the air blue in my car, little bits of spit forming at the corners of my mouth with anger and frustration... it's not healthy for my blood pressure I tells ya.
When there was too much background chatter and laughter in the office yesterday, I just wanted to stand up and scream "Shut up! Shut up! Shut up!" because I couldn't concentrate - but again I know this is down to feeling more tired than normal. The worrying thing for me is, I have no idea why I feel this tired. I just hope with the weekend coming up and the possibility of being able to stay in bed a little longer will make me feel more normal again. So to those people who know me, if I snap a little easier, I do apologise. Don't take offense, it's just me bein' knackered!
Now to try and stay awake at my desk for the whole day...