Hi folks,
When I tune either my electric, or acoustic guitar I’ve noticed if its out of tune its always above (sharp) of the where it should be and I have to tune down to the correct note.
I find this weird? Should it not be that if the strings go out of tune they go flat (down from the correct note) not up the way?
I’ve noted this phenomenon too Gordon.
From minor research on this I think this is the notion I find most likely.
When you just pick up your guitar, the strings ain’t been vibrating. The strings are cold so to speak.
As you start to play, the strings are vibrating. They start to warm up, literally because of this vibration. As they warm up, they get more pliable due to the heat, they expand.
So, ya tune up your guitar to these warmer strings, that have been vibrating while your playing for a while.
So then ya set your guitar down and don’t play till the next day. The strings go colder compared to when your playing and the strings are vibrating.
As they go colder, they contract due to being cool. As the go cold and contract, they go sharp.
Hence, the next time ya pick up your guitar, it’s slightly out of tune to sharp as the strings have contracted due to cooling.
So ya tune down to pitch.
As ya play they warm back up again and ya may need to retune as the 1st tune of the day is to them cool strings. As they warm, they go flat and ya gotta tune up just a bit to compensate.
The cycle repeats.
I could be incorrect, but I’ve for sure noted just what your talking about. This is the best explanation I’ve heard as to why this might happen.
Maybe others will chime in and have a different idea. And I’m for sure open to other ideas as I find it strange myself. Why sharp? My comment is what I think is going on.
edit, this phenomenon happens to both my acoustic and electric guitars. It happens to all (3ea.) to them.
I am not sure, but I made my own observations and drew a conclusion for my own case.
Here, the guitar is sharp or flat and I put it on the temperature/humidity change after when it has been tuned.
But you may be right that it is more often sharp when I pick it up than flat.
I find it a bit hard to find an explanation for it, because I pick it up and tune it instantly normally. There’s not really time for the guitar to heat up. Only while playing for a long time, but I almost never have to retune it after that first tuning upon picking it up.
Maybe it’s also a longer term thing? Like the inside rooms becoming dryer in winter and more hunmid in summer, so during a period, you have the guitars mostly flat and on the opposed period sharp when leaving them alone for a longer time?
It is definitely not the case with my nylon stringed instruments - here, they are more often a bit flat rather than sharp, probably because the material still lengthens all so very slightly over time when being under tension (as opposed to metal).
Haha, maybe there are gnomes hidden somewhere in a secret place around your house and they love to pick up your guitar when you’re away and play it? Because being little, they prefer the sound a bit on the sharp side 
I am sure there are multiple factors involved, but I suspect that the issues is temperature and humidity. Constant detuning (the opposite of what you have) is probably more of an issue with cheaper tuning machines or overall structural instability congruent with lower end guitars as noted below.
Whether Acoustic or Electric the wood will expand/contract based on the temperature and more importantly humidity. Metal will react to temperature, but my guess is that the temperature where most people keep their instruments does not have significant fluctuations, whereas humidity is harder to control. Wood reacts to both temperature and humidity, taking on more or less water weight. If you find your instrument sharp, it is likely that the humidity has increased since you last tuned and the guitar has grown, but the strings did not and are stretching to fit the “larger” instrument.
It is not an absolute rule, but higher end instruments tend to have better tuning stability. If your instruments are on the lower end you should expect to tune more regularly even in relatively stable humidity environment. I mention this only because expansion / contraction of the wood (resulting in constant tuning) can also cause long term problems with glue/binding, cracks etc. I.E. contestant tuning might be a symptom of a longer term issue. Cheaper guitar… maybe not a problem you can resolve, but higher end instrument, you might consider doing more to create environmental consistency.
Similar to Domi, I find I nearly always tune before I start playing. I don’t track whether it is sharp or flat, but anecdotally I think it is probably 50/50 and not typically more than about 5 cents.
Hope that helps. If not I had fund brambling anyway 
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I think humidity is the most likely explanation. Your guitar is absorbing moisture, causing the wood to expand, and the strings to go sharp.
In my part of Canada, my guitars go sharp in the summer when it is more humid, unless I put them in a bag with damp trap closet dehumidifiers.
In the winter, they go flat when the heat comes on, unless I humidify them.
In my case, my acoustic became almost unplayable in the humid season, because the action got so high.
I have tried to work out the answer to this. I think it’s just due to sort term (diurnal) temperature changes. I haven’t tested it. It’s just based on observations. But I noticed that my guitars are more out of tune when the temperature is fluctuating more between day and night and between warmer and colder days. Most guitars have some sort of lacquer especially electrics and I am very sceptical about short term changes in humidity affecting this as I doubt that much humidity can be absorbed so fast by by the wood in such a short time in significant quantities. But I know that many people on this forum pay a lot of attention to humidity (judging by the number of threads on that topic). I’m not saying humidity isn’t important but I think it’s only going to affect guitars over a longer period. However, contraction of materials during cooling and expansion during warming happens very fast. After all, that’s the principal in which a traditional thermometer works. So I think it’s the most likely explanation.
I haven’t done an experiment. A replicated experiment world require having several rooms of different temperatures and several guitars receiving different temperature treatments -could be an excuse to buy lots of guitars for a serious case of GAS
However my observations were that when there has been a big daily temperature change then the guitars were quite out of tune. Some a lot more than others. I have a sort of control for these observations, admittedly with only one guitar. I have a guitar in Bogota, Colombia as well as in Switzerland and France. The temperature in Bogota in the apartment is incredibly stable day, night and over the year with very small temperature fluctuations and that guitar stays in tune amazingly well and needs hardly any tuning when I pick it up. It’s also a really really cheap guitar - a Strat copy about the cheapest you can get. I’m talking in the $50 range.
Speaking as somebody in the uk I would have thought temperature of the strings, in particular changes in temperature might be a significant factor. For instance my acoustic guitar can be perfectly in tune in the house before I leave to go to the guitar club, but needs retuning when I get there where the temperature is different. I am not the only one, everybody tunes up.
Just my observations.
Michael
Thanks everyone, wow some very well researched, and interesting points. It seems likely that due to where I live; the west coast of Scotland, where rain is not scarce that change in temperature and/or humidity is a factor in this.
Thanks everyone! All your feedback is greatly appreciate!
Another possible cause of this is that the strings have excessive friction through the nut. You tune the guitar UP (from flat to correct), but because of excess friction, the string has slightly higher tension between the nut and the tuning peg than it does between the nut and the bridge. Over time, the excess tension behind the nut normalizes with the correct tension on the other side, which causes the string to be slightly more tense = sharp.
Try putting some pencil lead (actually, it’s graphite) into your nut slots and see if this helps. If the nut slots are too narrow for your strings, you can have them filed slightly wider.
Hope this helps,
Ed
We’re getting into autumn weather now: cool and low humidity. Yesterday was a fluky day with warmer temps (74°F) and higher humidity (61° dew point.) When I started using my strat-type, it was slightly sharp. It’s generally rock-solid in staying in tune. Clearly weather related.
100% weather conditions, usually humidity and temperature changes are both likely to affect your tuning, especially where you are in the UK, I’m in the West Midlands so it’s not as bad here 
Thanks DarrellW, nowhere is as bad for rain/ humidity as the West coast of Scotland:-)