Hi Camille! I hope you're doing well! While working on the song "Anyone" by Demi Lovato, I was only able to really hit the low notes in the verse (going down to an F#3) when I first woke up in the morning, and after doing the full soprano warmup (which ends in high head voice!), I wasn't able to reach those low notes anymore. I had to take a nap and then wake up again to be able to hit those notes! Do you have any suggestions (I.e.: doing a separate low note warmup specifically before singing lower notes) to help combat this? Thank you!

You can certainly try that, Vivian! You can also incorporate exercises that take you from high head voice back down to low chest voice: something wide-range. That way, your vocal folds have stretched out but are getting the chance to relax back as well. 

(For extremely low notes, though - that's common. Low notes are always easiest when your voice is totally relaxed and not warmed up at all).