Without necessarily lying, it is also possible to simply focus on how bad you felt : "I'm not sure if it was the flu, but my head just felt like it was going explode, good thing I'm better now"

Not down-voting, but I don't think lying is a good idea.

@MisterPositive The beauty of this response is it is not full-blown lying. I'd even stretch it to say it is not lying at all, it is just giving minimal details or graceful spin on a sensitive, personal topic which the OP has the right to keep private to a large extent. What I mean is, mental health issues being 'in your head', saying you had a headache (a somewhat ambiguous ailment and suffering related to your head) is perfectly legit.

In a perfect world nobody would ask so you would not have to lie, as this is clearly protected personal data. In this world you can not be sure you are safe from discrimination which I personally find to be the greater evil then the slight dishonesty. But of course, everybody has to use their own moral compass.

I find it odd that so many people are against a harmless little lie about what ailed you... Nobody can actually ever find out what it was you were out of the office for, so how can telling them you had a terrible headache or something ever 'come back to bite you'. But I don't get migraines or headaches on any regular basis, like you, but I have had a random stress headache attack that left me incredibly nauseous and bedridden for a few hours, so they can definitely just come about, especially if stressed!

@ Hanky Panky: thanks, corrected it! @Andrew: The beauty of headache is, you don´t actually have to tell much more. You have surely at some time banged your head ... just imagine that pain stays for some hours. Or press your hands to your temples and bite your tongue, that´s about it. Now, you can´t concentrate, have to lie down, quiet, darkness, sleep ... that helps. That´s about all one can say about a random headache ...

@AndrewCheong you can always add on that every once in a while you get these nasty headaches, and that you just can't focus or function when they hit; that lays the groundwork for potential future days off if the issue rears its head again. If your coworkers express concern that it's serious, just tell them you've spoken to your doctor and the doctor said it's nothing to worry about, to just get some rest and ride it out when it happens.

@cr0 It absolutely is lying. It's not what "headache" means; after all, the whole point of this response is to mislead co-workers about the type of illness. In the circumstances it might be a justifiable lie - OP shouldn't be obliged to disclose this kind of stuff - but that doesn't make it truthful.

I don't think it's morally wrong to say "headache" in this case, but there are a couple of reasons it might be a bad idea. Co-workers who hear "headache" may want to discuss it, asking further questions or offering advice, at which point you either have to complicate the fiction or give the same "I don't want to discuss it" that this is trying to avoid. It also makes it that much harder if you ever want to give the truth later on - people are likely to look back and re-evaluate those "headaches", and that can impact trust.

@GeoffreyBrent fair enough, all good points!

Comment shamelessly cribbed from one on another answer, but "migraine" is a good way to say "very bad headache" that sounds medically bad enough to justify a day off. But if the OP's not happy saying "headache" as they don't know what one's like, then "migraine" has the same problem but worse. That said many people don't get migraines, and those that do can experience them quite differently, so it might be possible to blag it with a bit of research beforehand.

@Geoffrey Brent: I don´t really see what there would be to discuss about a sporadic, stress-induced headache. I have that too once or twice a year and if I do I stay at home as I can´t concentrate on my programming. Never had to explain further. Full-blown migraine is another thing though - I would stay away from this. I fully agree that this is morally edgy and OP will have to decide for himself if he´s up to this.

@Daniel then you have sensible co-workers and you are lucky! (I do too.) But some people take this sort of thing as a challenge - "have you tried chiropractic? Have you tried a gluten-free diet?" and if OP 's aren't willing to leave it at "I was sick", they may be the latter kind.

+1 especially for if you just don't tell anything, everybody will suspect their own thing and it can start rumors., it's surprising that several answers don't address that at all. A better alternative to headache could be using the best approximation of the real situation that the OP is willing to afford. If it was me I'd consider, in this order: stress, sleepless night, need to recuperate mental energy.

This is a great answer (and I totally disagree that a white lie can be harmful in this situation). My go-to excuse is usually "some stomach bug". Just about everyone in the 1st world can sympathize with restaurant-induced diarrhea and or nausea... plus you are guaranteed not to get follow-up questions if you even hint at gastrointestinal distress.

everybody will suspect their own thing and it can start rumors : I have never worked in a place with such ridiculous busybodies for co-workers! As a developer, the people I've worked with are generally too busy and involved with what they're doing to get into such things. I pity anyone who works in such an environment.

"But, if you just don't tell anything, everybody will suspect their own thing and it can start rumors." Wow where are you working that people are guessing and rumor-milling about an occasional 1-day absence? That sounds like an incredibly toxic environment.

They wouldn't be busybodies guessing about a1-day absence, they would be normal people guessing about why the person is dodging the question "what was it".