To be honest, I have also relied on coffee during busy periods in the past. While it provided an immediate wake-up call, it eventually led to heartburn and sleep disturbances. It was, in a sense, a counterproductive effort. The advantages of diet coffee are clear: caffeine stimulates the sympathetic nervous system, which temporarily aids energy expenditure and can slightly suppress appetite. As a light starting point, it is not a bad choice. However, there are precautions. From a Traditional Korean Medicine (TKM) perspective, coffee has a strong 'heating' property. Those with Spleen Deficiency (脾虛, Bi-Heo)—a state where the digestive system is weak and prone to swelling—may experience indigestion as caffeine places a burden on the stomach. Furthermore, those with a constitution prone to Phlegm-Fluid Retention (痰飮, Dam-Eum) may experience heart palpitations or anxiety due to excessive caffeine. Ultimately, the question isn't 'what is good,' but 'what is my body's current state.' Rather than simply fasting or using stimulants, a healthy management plan that prevents the yo-yo effect should involve normalizing metabolism and clearing Blood Stasis (瘀血, Eo-Hyeol). I recommend deciding after considering whether your body is currently capable of handling caffeine, or if you first require rest and constitutional supplementation.