> If your cycle is 7 days (which it isn't, but which I initially understood it to be), you will want to make sure that, with an experiment running for 2N weeks, you take the LSD-spiked water exactly N times on Monday and N times on Thursday, rather than relying on luck to get close to that.

I see. So you were suggesting throwing in a categorical variable for the day of week and including that as a additive covariate to control for fixed-effects on days. I haven't actually run into any of my experiments depending on day of week to any important degree, but I still try to avoid 7-day blocks to make that less of an issue. It's not like there's usually an advantage to running in a weekly block, so better safe than sorry.