Plant droop in your situation is likely one of the following:
1. Temps are too high- your leaf edges will be standing up as well if this the primary issue.
2. Over and/or under feeding- this relates to nute levels, not frequency of watering. If the solution is too high, they will droop, if it is too low they will droop, but for different reasons.
3. Over and/or under watering- if the balance is off, the plants will have trouble adjusting & will likely droop at each application/withholding.
4. Humidity & air circulation- if is very dry or very damp, they will droop. Tied to this is something called vapor pressure deficit (vpd), but that is just too much info.
Likely it is a subtle overlap of the combination above. If they are otherwise healthy, it likely 1&4 in play. If you noticed color and/or growth abnormalities, it is often 2&3. Not to say any combination isn't possible... The 1-4 isn't so bad, you could easily just live with it. The 2-3 needs attention, especially if 1-4 are contribution factors. Pretty self-explanatory on what to do, depending on what you are seeing. You can further tie it down based on timing. At night, mid-day, after a watering, etc... Best.