Cycle of sleep. Abstract image of a clock mechanism and a woman asleep, depicting the biological cycle of sleep. Many biological processes are believe

---

Cycle of sleep. Abstract image of a clock mechanism and a woman asleep, depicting the biological cycle of sleep. Many biological processes are believed to take place under the direction of an internal body clock mechanism. Sleeping and waking, for example, are based on a daily or circadian (24-hour) cycle and are regulated by hormones such as melatonin and cortisol.

---

Size: 3257px × 4408px
Photo credit: © OSCAR BURRIEL/SCIENCE PHOTO LIBRARY / Alamy / Afripics
License: Licensed
Model Released: No

Keywords: alga, algae, aquatic, background, biological, biology, blue, botanical, botany, ccac, collection, contrast, culture, dic, differential, dissilens, flora, freshwater, hyalotheka, interference, light, material, micrograph, microorganism, nature, photomicrograph, single-celled, unicellular