Chronic Stress, Sleep & Circadian Rhythms

Welcome to Lesson 15 of the Human Biology Masterclass — the final lesson of Section 3: Control and Communication. In the previous lessons, we explored the nervous system, the brain, and the endocrine system. Now we bring these systems together to understand what happens when modern life repeatedly challenges the body's ability to adapt. In this lesson, you'll learn: • What chronic stress is and what the concept of allostatic load means • How repeated stress can affect the cardiovascular, immune, metabolic, and nervous systems • How prolonged stress is associated with changes in brain function and biological ageing • Why sleep is an active biological process rather than simply passive rest • How sleep deprivation affects cognition, mood, cardiovascular function, immunity, and metabolism • Why insufficient sleep is associated with changes in appetite regulation and metabolic health • What the glymphatic system is and how sleep may contribute to brain waste clearance • How circadian rhythms regulate daily patterns of physiology and behaviour • The role of the suprachiasmatic nucleus as a central circadian clock • How environmental light helps synchronize the body's internal clock • Why consistent sleep and wake timing can support healthy circadian regulation • How caffeine, evening light exposure, bedroom environment, and pre-sleep routines can influence sleep • Why chronic stress, sleep disruption, and circadian misalignment can interact with one another Stress is not inherently harmful. Acute stress is an important adaptive response that helps the body respond to challenges. The problem arises when stress becomes persistent and recovery is repeatedly disrupted. Sleep is equally important. During sleep, the brain and body remain biologically active, supporting processes involved in memory, emotional regulation, metabolism, immune function, and physiological recovery. Finally, circadian biology reminds us that human physiology is influenced not only by what we do, but also by when we do it. What's Next? In Section 4, we move into Transport and Defense. Next lesson: Lesson 16 – The Heart & Circulatory System Continue the complete Human Biology Masterclass through the playlist on Human Science Academy. Subscribe to Human Science Academy for educational content covering human biology, anatomy, physiology, neuroscience, endocrinology, nutrition science, exercise science, metabolism, and human health.