Home / Glossary
Words worth knowing.
The sleep and stress terms you meet in our Journal and across the internet, defined in plain language and linked to the research behind each one.
- Adrenal fatigue
- A popular label for tiredness blamed on worn-out adrenal glands. The Endocrine Society states there is no scientific proof that it is a true medical condition. [23]
- Brief arousal
- A short shift toward lighter sleep or waking that you may not remember. Brief arousals happen in healthy sleepers, are part of normal sleep and become more frequent with age. [6]
- Circadian rhythm
- The body’s roughly 24-hour cycle. A master clock in the brain is kept in step with the day by light signals from the eyes, which is why light at night can shift sleep timing. [17]
- Cortisol
- A steroid hormone made by the adrenal glands that follows a daily rhythm. In healthy adults it is lowest around midnight, starts rising around 2 to 3 AM and peaks in the morning. [8]
- Cortisol awakening response (CAR)
- The steep rise in cortisol over roughly the first 30 to 45 minutes after waking. Measuring it well takes saliva samples timed precisely from the moment you wake. [9][10]
- Cushing’s syndrome
- A condition caused by too much cortisol over a long period. Doctors diagnose it with a history, an exam and lab tests, usually more than one, because no single test is perfect. [24][25]
- Diurnal cortisol slope
- How steeply cortisol falls from morning to evening. Across 80 studies, flatter slopes were associated with poorer mental and physical health. That is an association, not proof of cause. [11]
- Insomnia
- Ongoing trouble falling asleep, staying asleep or both. Schedule changes, stress, caffeine, alcohol, nicotine, screens near bedtime and clock-watching can all raise the risk. [1]
- Melatonin
- A hormone the body releases in the evening, ahead of sleep. In one study, reading a light-emitting e-reader before bed meant less melatonin and a later body clock than reading a printed book. [16]
- Sleep debt
- An informal way to describe the running gap between the sleep you aim for and the sleep you get across several nights. It is a pattern to notice, not a clinical measurement. Sleep debt calculator
- Sleep diary
- A simple daily record of bedtime, wake time, night awakenings, caffeine, light and mood, kept for a week or more. It turns a vague feeling into a pattern you can show a clinician. How to keep one