Central nervous system and peripheral clocks regulate the rhythms of life. Main zeitgebers are light-darkness, nutrition, and fasting cycles. There is a fine interaction among human body clocks, and different stimuli can affect and modulate biorhythms. Nutrition is one of these. Its role is still under investigation, especially in healthy status balance maintenance of critical and noncritical patients. For these reasons, a narrative review of the current literature on the definition of circadian clocks, their regulation, and the role of diet in their fine-tuning in critical and non-critical care patients was conducted. A search was conducted on PubMed and Medline for original articles, reviews, meta-analyses, and case series matching the following keywords and associations: enteral nutrition; circadian rhythms; chrononutrition; critically ill patient; personalized nutrition. A total of 1188 manuscripts were identified according to keywords. Subsequently, 579 articles were removed due to duplication, 208 due to non-English language, book chapter publications, and non-matching keyword combinations. The remaining 401 papers were checked for eligibility, and 170 manuscripts were accepted. Human central and peripheral circadian clocks are affected by several zeitgebers. Food micro- and macronutrients and time of administration can all affect peripheral clocks and interact with central ones. In critical and non-critical care patients, route and timing of artificial nutrition administration should consider the behaviour of different central and peripheral circadian clocks. This is also affected by inherited genetic biotypes. In detail, specific macronutrient patterns modulate peripheral clock gene expression and influence metabolic recovery in critically ill patients. In fact, timed and/or time-restricted setups of enteral nutrition administration (namely, timed enteral nutrition refers to administration according to circadian rhythms) show promising and yet incomplete data on their efficacy for restoring healthy status in the patients. Human circadian rhythms recognize nutrition as one of the main regulators. Both food composition and administration schemes can help to establish effective cycles in critical and non-critical admitted patients.
