Research from the Journal of Psychophysiology indicates that getting up immediately upon waking helps regulate the body's cortisol awakening response (CAR). This natural rise in cortisol levels in the morning provides energy and alertness, setting a biological rhythm for the day.
Source: Clow, A., Hucklebridge, F., & Thorn, L. (2010). The cortisol awakening response in context. International Review of Neurobiology, 93, 153-175.
Studies published in the Journal of Personality and Social Psychology show that willpower is highest in the morning and depletes throughout the day. By making the decision to get up immediately, you're leveraging your strongest willpower reserves, creating momentum for better decisions throughout the day.
Source: Baumeister, R. F., Bratslavsky, E., Muraven, M., & Tice, D. M. (1998). Ego depletion: Is the active self a limited resource? Journal of Personality and Social Psychology, 74(5), 1252-1265.
Research from the American Journal of Physiology demonstrates that consistent wake times help regulate circadian rhythms, improving sleep quality and daytime alertness. Getting up immediately (rather than hitting snooze) reinforces these natural biological cycles.
Source: Duffy, J. F., & Czeisler, C. A. (2009). Effect of light on human circadian physiology. Sleep Medicine Clinics, 4(2), 165-177.
Studies from organisational psychology have correlated early rising and immediate action upon waking with higher productivity levels and achievement. This first "win" of the day creates psychological momentum.
Source: Kühnel, J., Sonnentag, S., & Bledow, R. (2012). Resources and time pressure as day-level antecedents of work engagement. Journal of Occupational and Organisational Psychology, 85(1), 181-198.