bass goes out of tune?

[Calaverasgrande]

Strangest thing. I only noticed this when my band stopped practicing twice a week and cut it to once a week. When I tune my bass (EBMM Sterling) and leave it alone for 3-4 days to week it will go out of tune. But in an odd way I have never seen before. It goes up about 10-15 cents! Is this my truss rod being too tight? I bought this bass used about 6 months ago and like it a lot. However It has a few dings and the kid I bough tit from didnt seem to have much of a clue about bass care.

Any ideas guys?

[TGLbass]

Most all my basses go sharp after a week or so sitting in the cases. They're in my basement. It's not damp but it's about 70 degrees. The cases are standing upright. I don't think it's that odd. There's nothing wrong with the guitars. I use 45-105 Rotos. The guitars have done that for over 10 years that I can remember. It started happening in my other house. I'd never noticed them doing it before about 15 years ago.
I'm sure someone on here has a theory. I'm not worried about it because they've done it for such a long time and there's absolutely nothing wrong with them. I think it may just be temperature fluctuation.

[Pilgrim]

Ah! Physics works!

When you play, the strings get warm from your hands..and metal expands as it warms. When the bass sits, the strings cool and contract, often going sharp. Once you warm them up again, they expand and drop tune just a bit.

[Calaverasgrande]

Well, I been playing bass for 20-30 years and this is the first time I ever noticed it. Maybe my other basses had stiffer necks or...necks that could absorb the change in tension? If anything I usually bang my strings out of tune from playing hard and fast. So I am accustomed to my bass being flat on a couple strings (C and F usually) when I show up for practice. I also am used to going quite long between string changes when I am not on tour. So maybe thats it. I have recently been using dunlops I bought a crap ton of at GC on sale. So I change them every couple months.