@jimk849 - Another thing to look into might be a vitamin deficiency -- I know potassium deficiency can cause muscle cramps for sure (I used to wake up with TERRIBLE foot cramps as a kid and my mom would shove a banana down my throat for potassium -- I HATE bananas lol) and so can Vitamin D deficiency. I saw a podiatrist (foot doctor) who diagnosed me with a high arch and fit me with orthotonics for my shoes (these are different from insoles you would buy at a pharmacy) -- might be something to look into, especially since you mentioned you have flat feet. Electrolyte imbalances like low sodium (hence the pickle juice suggestion) and dehydration can also cause muscle cramps, as several others mentioned. Check your medications also, as some can cause/be associated with increased leg or muscle cramps -- I ended up in the hospital with unbelievable charley horses that left me unable to walk in high school, which was eventually blamed on Zoloft and/or another medication. If I'm recalling correctly (and I very well might not be!), when I would wake up with charley horses in high school, I eventually figured out that putting pressure on the ball of my foot with my heel off the ground and perpendicular to my toes helped (I'm trying to find a picture but having trouble) -- basically your toes and the ball of your foot are flat against the floor or hard surface (I would wake up napping during the day with the charley horses, and pushing against the couch arm worked miracles, but if your bed is against the wall or you have a *very* sturdy footboard that might also work) while the rest of your foot is straight up in the air and you're applying pressure/leaning forward or into it slightly; I actually still do this switching from foot to foot when I have foot cramps/pain.

For plantar fascitis ( @lisalucier and @johnbishop ), I bought a compression sock on Amazon for $7 that works wonders (can also be found at most pharmacies also) - https://smile.amazon.com/gp/product/B01ED7YVGY/ref=oh_aui_detailpage_o01_s00?ie=UTF8&th=1&psc=1. You can also buy a little ball massage roller thing to massage with before going to bed or when you wake up with pain; they look like spiky dryer balls, or even rolling a full water bottle back and forth under the foot might help.

If none of that pans out and you continue to have problems with some frequency, you may consider visiting a neurologist to look into any spinal/nerve problems, including dystonia and Parkinson's.

Finally, on the less likely/longshot end, but wanted to mention it just in case, might be some sort of early/unusual manifestation of restless leg syndrome.

Hope you find some relief! Good luck!