Hunger, Thirst, and Sleep

When the gasoline gauge in an automobile nears the "empty" mark, it acts as a signal to the driver to do something. In one sense, the gauge acts as a stimulus which disposes the individual to gasoline-seeking behavior. In a similar way, the feelings of thirst and hunger that we experience daily act as stimuli which dispose us to food-seeking behavior. This module deals with the mechanisms of the hunger and thirst drives.

As you read the text, try to answer the following questions.


When organisms are thirsty, they have a tendency to seek water. When they find water and drink, they are no longer thirsty, and the thirst drive is reduced. In order to find out more about the mechanisms involved in the thirst drive, researchers have tried to determine the conditions under which organisms drink.

Adolph (1941 ) performed an experiment to find out how dryness of the mouth relates to thirst. A surgical operation made it possible for water to be placed in a dog's stomach without passing through and wetting the dog's mouth. It was also possible for water to pass through the mouth but not into the stomach. The dog was deprived of water for a time and then offered a drink. The dog drank a normal amount of water, even though none of it passed into his stomach. After a short time, the dog drank again. The experimenters proved that dryness of the mouth has something to do with the thirst drive.

In the next part of the experiment, water was placed directly into the dog's stomach without passing through and wetting the mouth. When offered water immediately, the dog drank. If, however, the experimenter waited 15 to 30 minutes after water had been placed in the stomach, and then offered water, the dog refused to drink. It was concluded that in order for the thirst drive to be reduced, time must be given for water to be assimilated by the organs of the body. Hunger and thirst can be stimulated artificially

The thirst drive is under the control of the hypothalamus. When this area is stimulated artificially, an animal drinks. When the organs of the body need water, the hypothalamus is stimulated and it, in turn, brings about the drinking response.


The tendency to seek and eat food is the hunger drive. We get hungry in many circumstances. All of the following can make us want to eat: the smell or sight of food, the sound of a dinner bell, even passing our favorite restaurant. Psychologists are interested in the mechanisms underlying the hunger drive. What happens internally that makes us desire food?

The hypothalamus has a great influence not only over drinking, but over eating behavior as well. In the hypothalamus, there are two nerve centers whose actions have opposite effects. If one of these, the feeding center, is stimulated, an animal will eat whether he is hungry or not. If the feeding center is removed, the animal will not eat. The other hypothalamic center is called the satiety center. The satiety center tells the organism when he has had enough to eat. Removal of the satiety center causes an animal to eat continuously and he will grow far beyond his normal size.

When we are subjected to certain stimuli, the hunger-producing center initiates the eating response. When we have eaten enough, the satiety center tells us to stop.

Many of the stimuli that tell the hypothalamus that we are hungry originate in the organs of the body. If the nutrient level of the blood is too low, the hypothalamus is alerted and the feeding center, initiates eating behavior. External stimuli can also initiate eating behavior. The sight, sound, and even the thought of food initiate impulses that eventually reach the feeding center in the hypothalamus. Specific hungers are stimulated by specific deficiencies

Deficiencies in the body produce a specific hunger for certain foods. Scott and Verney (1949) showed that when an individual is deprived of food containing vitamin B, he develops a marked preference for foods containing this vitamin. Women who are pregnant apparently need a great deal more of certain nutrients than they need normally. There are instances in which women have eaten plaster from walls to provide their bodies with minerals not available to them in any other form. As might be expected, there are specific satiations as well as specific hungers. Organisms fed a high-protein diet will tend to avoid high protein foods if given a choice, even when hungry.

When human beings are kept from eating, they show definite signs of psychological impairment. In one study (Keys, et a/., 1950), 32 men were kept in a state of semi starvation for six months. Among the effects noted were weakness, irritability, loss of interest in sex, and a general obsession with food. This obsession became so strong that it interfered with their intellectual performance.


Contrary to previous evidence, it now appears that the internal clocks for sleep in humans works the same way as in other animals (Goode, 1999). Further, all humans appear to adjust their sleep and wake cycle to the same 24 hour cycle as in the solar system. This circadian clock is located in the SCN (suprachiasmatic nucleus), a small structure in the hypothalamus and picks up information from the retina and various brain structures. There do not appear to be wide differences among individuals, nor does it appear true that older persons require less sleep.

In a new study reported in Science June 25, 1999, Charles Czeisler and colleagues at a Harvard affiliated hospital, studied 24 subjects who lived in a laboratory for three to four weeks where there was no natural light. Their sleep wake cycle was moved four hours later each day, thus increasing their sleep/wake cycle. But apparently their internal clocks, as measured by body temperature, never varied from the 24 hour clock, even though they never had external cues. The results were the same for men and woman and older and younger subjects.

These findings have important implications for late night shift workers and travelers. Using bright lights can serve to adjust this cycle.


Now test yourself without looking back.

1. Wetting a dry mouth:
a. reduces the thirst drive as efficiently as drinking.
b. reduces the thirst drive, but only for a brief time.
c. has no effect at all on the thirst drive.
d. completely eliminates the thirst drive.

2. The thirst drive is most effectively reduced after water has been assimilated by_____________________

3. Drinking behavior is elicited by stimulating a structure in the brain called the__________________

4. The center in the hypothalamus that initiates eating behavior is called the _______________________ center.

5. When a nerve center was removed from the hypothalamus of an experimental animal, the animal ate until________________________he was much larger than normal. The center removed was the_____________________ center.

6. When a person needs food, the hypothalamus is signaled that the blood is______________________

7. What is shown by experiments on specific hungers?__________________

8. What might be three psychological effects of semi-starvation?____________________

9. The hypothalamus may be signaled to initiate eating behavior by:
a. a low nutrient level in the blood.
c. the sight of a steak.
b. a dinner bell.
d. a picture of food.



Water was placed directly into a thirsty dog's stomach without passing through and wetting his mouth. Immediately thereafter, given water, the dog drank as much as he would have required had water not been placed in the stomach. This experiment indicates that:
a. dryness of the mouth has nothing to do with the thirst drive.
b. dryness of the mouth has something to do with the thirst drive.
c. the amount of water in the stomach has nothing to do with the thirst drive.
_________________________________ 7

The above experiment was repeated with one difference. This time the experimenter waited 20 minutes after water was put into the dog's stomach before offering water. The dog refused to drink. This result indicates that:
a. the thirst drive was reduced after the water had been assimilated by the dog's internal organs.
b. the thirst drive is not affected by water in the stomach.
d. the thirst drive is controlled by mouth dryness.
_________________________________ 6

Drinking responses are mediated by the hypothalamus. When a particular center of the hypothalamus of an animal is stimulated, the animal______________________________________
___________________________________________ 3

The thirst drive is most effectively reduced only after water is assimilated by the body's organs. If a thirsty animal's esophagus is diverted so that no water enters the stomach, which of the following is likely to happen?
a. He will drink normally and the thirst drive will be reduced.
b. He will refuse to drink.
c. He will drink a normal quantity but, after a short time, he will drink again.

The hypothalamus contains two nerve centers: a satiety center and a feeding center. These centers have opposite effects on the hunger drive. If the satiety center is stimulated the animal will behave as though he were not hungry. An animal will continue to eat until he grows abnormally large if the_______________________ center is continually stimulated.

If the satiety center is stimulated, the animal will _____________________
_____________________________________________ 2

If the blood stream does not carry enough nutrients to the organs of the body, the organism will attempt to eat. The eating response is controlled by the hypothalamus. Therefore, in a hunger situation, the hypothalamus must in some way be signaled by:
a. the amount of food in the stomach.
b. the nutrient content of the blood.
c. the feeding center.
d. the satiety center.

1 c 5 feeding
2 not eat
6 a
3 drinks
7 b
4 b
8 satiety


Experiments on specific hungers show that organisms tend to eat foods that contain nutrients needed by the body. If an animal has a great deficiency of salt, he will choose foods that have_________________________________________4

Extended hunger has a drastic psychological effect. It has been shown that human beings become weak, irritable, disinterested in sex, and generally obsessed with food when:
a. they suffer hypothalamic damage.
b. their stomachs are removed.
c. the satiety center is stimulated.
d. they are semi-starved.
______________________________________________ 2

There are many different stimuli that signal us that we should eat. All of these signals eventually reach the hypothalamus which has direct control over the eating behavior. Which of the following stimuli might act as a signal to the hypothalamus?
a. A table set for dinner
b. The nutrient content of the blood
c. A thick steak
d. A dinner bell



2 d
4 high salt content
5 a b, c, d


1. Under what physiological condition is the thirst drive most effectively reduced?____________________________

2. An experimental animal was stimulated in an area of the brain. He immediately began to drink. The area stimulated was probably the _________________________________________

3. When an animal's feeding center is stimulated, what happens?_________________________

4. Describe the appearance of a laboratory rat that had its satiety center removed two weeks ago___________________________________________.

5. A physiological condition that alerts the hypothalamus when the body needs food involves________________________________

6. A rat was deprived of vitamin C for a three-week period. If the rat were then offered a choice of many different foods, what kind would he choose?____________________________

7. The rat in question 6 is said to have a _______________________________ for certain foods.

8. Lowered sexual drive and impaired intellectual ability may result from______________________________________


Unit 9 Table of Contents

January 8, 2007