CLINICAL FOCUS 13-1 DOING THE RIGHT THING AT THE RIGHT TIME

13-1 A CLOCK FOR ALL SEASONS

ORIGINS OF BIOLOGICAL RHYTHMS

BIOLOGICAL CLOCKS

EXPERIMENT 13-1 QUESTION: IS PLANT MOVEMENT EXOGENOUS OR ENDOGENOUS?

MEASURING BIOLOGICAL RHYTHMS

FREE-RUNNING RHYTHMS

ZEITGEBERS

CLINICAL FOCUS 13-2 SEASONAL AFFECTIVE DISORDER

13-2 NEURAL BASIS OF THE BIOLOGICAL CLOCK

SUPRACHIASMATIC RHYTHMS

KEEPING TIME

PACEMAKING CIRCADIAN RHYTHMS

RESEARCH FOCUS 13-3 SYNCHRONIZING BIORHYTHMS AT THE MOLECULAR LEVEL

PACEMAKING CIRCANNUAL RHYTHMS

COGNITIVE AND EMOTIONAL RHYTHMS

13-3 SLEEP STAGES AND DREAMING

MEASURING HOW LONG WE SLEEP

MEASURING SLEEP IN THE LABORATORY

STAGES OF WAKING AND SLEEPING

A TYPICAL NIGHT’S SLEEP

CONTRASTING NREM SLEEP AND REM SLEEP

CLINICAL FOCUS 13-4 RESTLESS LEGS SYNDROME

DREAMING

WHAT WE DREAM ABOUT

13-4 WHAT DOES SLEEP ACCOMPLISH?

SLEEP AS A BIOLOGICAL ADAPTATION

SLEEP AS A RESTORATIVE PROCESS

SLEEP AND MEMORY STORAGE

13-5 NEURAL BASES OF SLEEP

RETICULAR ACTIVATING SYSTEM AND SLEEP

NEURAL BASIS OF THE EEG CHANGES ASSOCIATED WITH WAKING

NEURAL BASIS OF REM SLEEP

13-6 SLEEP DISORDERS

DISORDERS OF NREM SLEEP

CLINICAL FOCUS 13-5 SLEEP APNEA

DISORDERS OF REM SLEEP

13-7 WHAT DOES SLEEP TELL US ABOUT CONSCIOUSNESS?

Doing the Right Thing at the Right Time

We have all heard this advice: Maintaining regular sleeping and eating habits contributes to good health. Scientific evidence supports this good advice.

We humans are diurnal animals (from the Latin dies, meaning day): we are active during daylight and we sleep when it is dark. This circadian rhythm is the day–night rhythm found in most animals. It evolved to maximize food acquisition during the day, when we see best, and to minimize expending our energy stores by sleeping during the night hours, when do not see well.

Today our environment allows us to intrude on our circadian rhythm in two ways. Artificial lighting allows us to extend our waking hours well into the night and into sleep time. Handy food sources—often easily metabolized high-calorie foods—allow us to eat whenever we want.

Together these intrusions into our natural circadian rhythm contribute to a combination of medical disorders known as metabolic syndrome that collectively increase the risk of developing sleep disorders, cardiovascular disease, and diabetes (Lajoie et al., 2015).

The roots of metabolic syndrome lie in disruptions of our biological clock, the neural system that times behavior. One clock controls sleep–waking. Another controls the functioning of body organs related to feeding, such as the liver, pancreas, and gut. The clock that controls sleep–waking responds to light, whereas the clock that controls feeding responds to eating. Their activity is reciprocal: disrupting one disrupts the other.

Irregular sleep and meal schedules thus change the synchrony of biological clocks. Metabolic rate, plasma glucose, and pancreatic insulin secretion can slow down or speed up at inappropriate times, contributing to obesity and diabetes. Prevention and treatment for obesity and diabetes can include doing the right thing at the right time when it comes to sleep schedules and mealtimes.