Question: what essential oil to diffuse?
Answer: Fresh & Happy. Lemon, Grapefruit, Wild Orange. I love these scents for daytime, not so much for sleeping. ... 
Warm & Rustic. Clove, White Fir, Cassia. For that fall-winter mood (which for me kind of lasts all year long as far as which scents I naturally gravitate toward). ... 
Neutral & Clean. Purify, Bergamot, Eucalyptus.