Will Exist find correlations across multiple days?

In short: yes.

If a correlation between two attributes isn't found for the same day, Exist will check for correlations between attributes across two consecutive days. For example, does tracking more alcoholic drinks (or using the tag "alcohol") affect your sleep the following night? Or, does more time working out affect your mood the following day?

When entering data into Exist, we recommend you track things as they happen—i.e. if you drink alcohol today, track those drinks or add the "alcohol" tag today. Exist will check whether this correlates with things that happen tomorrow, so you can track your data accurately without worrying that you won't find the correlations you're looking for.