You are feeling very sleepy...

I'm tired. And I'm tired of being tired. It's just so tiring.

It's the old radiation fatigue. It's not going away, and it's becoming a nuisance.

It seems to have been hanging around me like a lead cloud for weeks, like I'm swimming in jelly while wearing an Aran sweater. And the haircut isn't making me any more streamlined.

It will go away, of course. Probably. But I don't know when, and the unpredictability is the worst bit. I can't plan my day.

While I was still being zapped on a regular basis, I knew what to expect. I'd get up feeling fine, start to flag a bit by lunchtime, have a nap and a snack, feel better, start to flag a bit again in the afternoon, then have another nap to restore me for the trip to hospital to top up on sub-atomic weariness for the next day. All well and good; I could work round it.

This, though, is just awkward. Most days I still get up quite energised, but where things go from there is anyone's guess. Most days I have three or four hours clear before it starts to creep up, but on others I'm feeling it before I've even left the house.

It starts in the muscles of the arm, a little like the onset of flu but without the actual ache, and moves quickly into a slight dizziness, a sense of dissociation from my surroundings that means I'm no longer quite able to focus, sometimes accompanied by lethargy that feels like depression. It's a little like being drunk – and if you don't see what's so bad about that, to paraphrase Douglas Adams: ask a glass of water.

None of this is particularly debilitating, and I can push through it quite easily. But doing so comes with the knowledge that there's a debt to be paid later.

Sometimes I can stave off repayment. Eating helps: I think low-carb, high-protein stuff like nuts are most effective, sugary things seem to produce a false high. Coffee may or may not help: that may just be an existing addiction. Drinking lots of water certainly helps.

But, like running up a credit card, it doesn't really go away. Eventually, I need to tackle the bill. And all that really works is sleep. It won't really be cleared until I do that.

Which isn't so bad. Sometimes I need a couple of hours, but mostly half an hour will drag me back to normal; sometimes ten minutes is enough. Problem is, I started back at work last week, and while it's good to be back on project and with a daily purpose again, I can no longer merely retire to my couch. I'm in a place of business, among colleagues, and nodding off against the ergonomically-designed desks isn't a good look in a modern working environment.

So I push through it and rack up the debt, and eventually it hits. Usually in the middle of something I'm trying to concentrate on, occasionally mid-sentence. It's like the cold shock of a bucket of water, but with reverse effect, slamming me into torpor. And at that point I have to go home.

Fortunately, my employer is flexible enough to allow me to do that. I just have to get up, go, and try not to fall asleep standing in the bus queue. But it would be nice to know when it was going to happen. So I could guarantee to be at meetings, for instance.

Take last week. Monday, my first day back for three months, wasn't too bad. I got what I needed to do done, and made it to mid-afternoon before bailing out. Tuesday I was in hospital for most of the day, so no problem – they're completely happy with patients falling asleep, except when they're shoving sharp things into you, which they like you to experience and feed back on. Wednesday, though, was awful: I barely made it past lunchtime, and though I pushed on as far as I could, I went home with the determination that the next day I would be in only for long enough to explain that I'd come back too early, I wasn't ready yet, and would have to get my sick line extended.

On Thursday, of course, I felt fine. Great, in fact; made it through to after 4.30pm before feeling that I should probably call it a day. And that's been the thing since: some days are tremendous – today was fine, thanks, did my first full shift since returning in fact – but others are bloody awful. Yesterday I felt so bad I was in for only a couple of hours.

Some kind of pattern would be nice, but all anyone can really tell me is that this is all perfectly normal.

I'd been told to expect the fatigue, of course. Two weeks, three weeks, six to 12 weeks of it, apparently. It could kick in during treatment, I was told, or weeks later. No one seemed very sure, but I was certainly warned. It varies from person to person, it seems. No-one knows how it will take each individual. Fine. I just wasn't expecting inconsistency with it.

I was definitely feeling it in the last weeks of my treatment, for instance, and felt increasingly better each week as it stopped. You'd expect that to continue, and it appeared to: when I got to Gran Canaria, I felt not at all bad, and I returned refreshed, relaxed and ready for anything.

Except, that is, for the decline which kicked in within about three days of returning to Scottish soil, and really hit just in time for getting back to work. My timing, as always, is impeccable.

Still, things are otherwise going pretty well. I started adjuvant chemo this morning, which is double the dose I was on previously, with no apparent side-effects. My platelet count is apparently normal, so I can continue to drip-feed the Beatson's pet vampire without running out, and my white cells are doing whatever it is they're supposed to. Plus, today was simply beautiful – a perfect, fresh spring morning, pale blue and crisp. And that made me feel fantastic.

Little things, it seems, do mean a lot.

I'm going to bed now.