Rejuvenation is a feeling of mild to extreme cognitive refreshment which is felt during the afterglow of certain compounds. The symptoms of rejuvenation often include a sustained sense of heightened mental clarity, increased emotional stability, increased calmness, mindfulness, increased motivation, personal bias suppression, increased focus, and decreased depression. At its highest level, feelings of rejuvenation can become so intense that they manifest as the profound and overwhelming sensation of being "reborn" anew. This mindstate can potentially last anywhere from several hours to several months after the substance has worn off.
Rejuvenation is most commonly induced under the influence of moderate dosages of hallucinogenic compounds, such as psychedelics and dissociatives. However, it can also occur to a lesser extent under the influence of entactogens, cannabinoids, and meditation.
There are a number of legitimate scientific studies which have concluded that long lasting mental benefits can occur after a drug has been taken. For example, it has been demonstrated that ketamine, even if taken in small doses, is effective for patients suffering from chronic depression and bipolar disorder. Studies have shown   that the effect of the drug is immediate or within 2 hours and consistent in relieving a patient’s depressive and/or suicidal symptoms, lasting up to 3 days after a single dose.
A similar example includes a study which observed significantly decreased depression scores in terminal cancer patients six months after treatment with psilocybin.  An open-label study was carried out in 2016 in the UK to investigate the feasibility, safety, and efficacy of psilocybin in treating patients with unipolar treatment-resistant depression with promising results; although the study was small and involved only twelve patients, seven of those patients met formal criteria for remission one week following psilocybin treatment and five of those were still in remission from their depression at three months. 
- Ketamine Improves Bipolar Depression Within Minutes | http://www.medicaldaily.com/articles/10085/20120530/ketamin-bipolar-disorder-depression.htm
- Could A Club Drug Offer 'Almost Immediate' Relief From Depression? | http://www.npr.org/blogs/health/2012/01/30/145992588/could-a-club-drug-offer-almost-immediate-relief-from-depression
- Grob, C. S., Danforth, A. L., Chopra, G. S., Hagerty, M., McKay, C. R., Halberstadt, A. L., & Greer, G. R. (2011). Pilot study of psilocybin treatment for anxiety in patients with advanced-stage cancer. Archives of General Psychiatry, 68(1), 71-78. | https://doi.org/10.1001/archgenpsychiatry.2010.116
- Carhart-Harris, R. L., Bolstridge, M., Rucker, J., Day, C. M., Erritzoe, D., Kaelen, M., ... & Taylor, D. (2016). Psilocybin with psychological support for treatment-resistant depression: an open-label feasibility study. The Lancet Psychiatry, 3(7), 619-627 | https://doi.org/10.1016/S2215-0366(16)30065