You can put it in the fridge. Extraction is just weaker (than I like) at lower temperature with thermodynamics and everything.

At 12 hours, you have lighter extraction than 24 hours.

I think about it like chemistry: more interaction between solvent (water) molecules and solute (coffee grounds), more extraction. So adding any of these is more interaction and extraction:

• time
• temperature (hot molecules move more than cold ones)
• motion
• finer solute (finer grind)

One thing I haven't tried is adding motion. I'd like to maybe get a shaker or stirrer that I can leave on for the full brew.