See the Warm Up section of the Homeostasis lesson for the script of a partial interview with Michel Siffre.
Michel Siffre (born Jan. 3, 1939) is a French speleologist and scientist. As a child, Siffre was fascinated by space travel and decided that living in isolation in an underground cave would be the closest thing on Earth. At the age of 23, Siffre lived in an underground glacier in the Alps for 63 days. He did not have a clock or calendar. His body was allowed to regulate time on its own; Siffre still had urges to sleep and eat without the cues of night and day. He completed a longer isolation experiment a decade later (NASA sponsored). His experiments helped detail the human internal clock, and was useful to space and military organizations.
The human body is a dynamic system that responds to stimuli, or external or internal signals that cause a response. Through the response, the human body must maintain homeostasis, which is an internal balance our bodies must preserve in order to be efficient and healthy.
A common example of homeostasis is human body temperature. The average human body temperature is 98.6°F (37°C). When we get cold, our bodies shiver to warm up. When we get hot, our bodies sweat to cool down. If our body temperature is disrupted for an extended period of time, we will become ill and can possibly die. For example, prolonged high body temperature can lead to a fever. Depending on the severity, symptoms span from dehydration and fatigue to seizures, tissue and organ damage, and death. On the other hand, prolonged low body temperature can lead to hypothermia. Depending on the severity, symptoms span from drowsiness and confusion to frost bite and death.
One way our bodies respond to stimuli is called a positive feedback mechanism. In this case, our bodies respond to a stimulus by increasing a process until homeostasis is reached. Our bodies sense damage to skin when we get a paper cut and start to bleed. The response is to send specialized cell fragments called platelets to the site of the wound. The platelets will produce a clot that will prevent the loss of more blood. Once platelets show up to the wound site, it will attract more platelets to move in that direction so it can heal faster. Homeostasis is reached when we stop bleeding, the wound is closed with a scab or scar tissue, and new skin forms. This is an example of a positive feedback mechanism because it the stimuli elicits an increased response to achieve homeostasis.
Another way our bodies respond to stimuli is called a negative feedback mechanism, which is more common. In this case, our bodies respond to a stimulus by decreasing a process in order to maintain homeostasis. Our blood sugar (glucose) levels rise when we eat. As a response, our pancreas releases a hormone called insulin, which triggers cells to absorb the glucose. It also causes the liver to absorb glucose and stores it as glycogen—a form of blood sugar that is on “stand by”. This will result in a drop of glucose levels in the bloodstream. When glucose levels drop too low, our pancreas will release another hormone called glucagon, which causes our liver to release glycogen as glucose into the blood stream until we eat again. This is an example of a negative feedback mechanism because it the stimuli elicits a decreased response to achieve homeostasis.
Homeostasis is heavily regulated by our bodies so we can be efficient and healthy. Our biological clock is a genetic mechanism that directs cell activity so we can maintain some aspects of homeostasis. Humans have a biological clock that operates on about a 24-hour cycle. Our bodies take cues from the environment, such as light and dark, to run the clock, but studies have shown that humans can maintain a longer cycle even in the absence of environmental cues.
Looking at the roughly 24-hour cycle, we find that there are physiological events that take place around a particular time of day—almost like a set schedule. This cycle is called the circadian rhythm. The term circadian comes from Latin where circa means “about” and diem means “day”.
Ask students to think about the things that characterize sleep before showing them the list.
Humans are diurnal, meaning we are active during the day and sleep at night. Our sleep and wake cycle is predominantly controlled by environmental cues of light and dark. Refer to Figure 5. When we wake up, light enters our eyes and a signal is sent to the suprachiasmatic nuclei (SCN), a bundle of about 50,000 cells located in the hypothalamus of the brain of mammals. It is considered the “control center” of our circadian rhythm. When the SCN detects light (signaling daytime), it will direct many other areas of the brain to heighten alertness and lower sleepiness. When the SCN detects lowering levels of light (signaling nighttime), it will do the opposite. Light is the strongest external stimulus that affects our circadian rhythm. Other external stimuli include temperature, social cues, medications, exercise, and diet.
An example of how the SCN controls our circadian rhythm is by directing the production of melatonin, a hormone that causes sleepiness and lowered body temperature. Refer to Figure 6. When light enters our eyes in the morning, the SCN directs the pineal gland of the brain to stop producing melatonin. With lower levels of melatonin in the bloodstream, we feel more awake and alert. Later, light will decrease as we move into nighttime. With lowering levels of light, the SCN to direct the pineal gland to produce more melatonin. With higher levels of melatonin in the bloodstream, we feel sleepy. Since melatonin appears mostly at night, it is referred to as the “sleep hormone” and even the “Dracula of hormones”.
The production of melatonin as a function of light explains our circadian drive to sleep, but what about our homeostatic drive to sleep? We find that when we deviate from a set sleep schedule, we experience physiological changes. For example, if we just change from a day-shift job to a night-shift job, we will feel very tired after the first shift and have a strong homeostatic drive to sleep. The homeostatic drive to sleep is independent from the circadian drive to sleep because it is no longer affected by light, it is affected by the history of our sleep. Some of us try to stay awake and alert by drinking coffee, which contains the stimulant caffeine. Caffeine may temporarily suppress the urge to sleep but it will not eliminate our need to sleep. If this pattern of staying up late continues throughout the week, we might “crash” over the weekend in order to make up the sleep debt that was accumulated. When we make up for the sleep debt, we do not make up the exact amount of time that was lost over the week; instead we experience a deeper sleep. Scientists noted that people compensating for a sleep debt tolerate a higher arousal stimulus and can be more confused upon waking up.
This phenomenon of circadian and homeostatic sleep drive is displayed in the figure. The top graph shows a normal sleep cycle of a day-shift worker. Note that the day-shift worker starts to fall asleep when both circadian and homeostatic drive for sleep is the strongest; in other words, the drives are synchronous and the person can have a consolidated night of sleep. The bottom graph shows the sleep cycle of a new night-shift worker. Note that the circadian drive to sleep remains the same as the top graph because it is still light during the day and dark at night, therefore melatonin will still decrease during the day and increase at night. The homeostatic drive to sleep, though, becomes out of sync. We will feel a strong urge to sleep when we are required to be awake. Sleep will not be consolidated and will not feel as restful. Over the course of the week, a sleep debt can accumulate and the person may “crash” over the weekend. The less sleep we get, in terms of time and/or quality, the more we want to sleep.
Ask students to take the “Are You a Lark or an Owl?” quiz and then show them the scores. Discuss the factors that can determine whether someone is a lark or owl.
Part 2: Sleep Journal 1. Distribute a My Sleep Journal to each student and instruct them to write their name on the cover. 2. Go over the instructions for completing the My Sleep Journal over the course of the week (an instruction and example page are located at the front of the Sleep Journal): a. Write the “Day” and “Date” for each page (this will vary because you can choose when to start). b. Keep the My Sleep Journal and a pen close to your bed. c. When you wake up in the morning, fill in the remaining columns. Write what you did an hour before bed, what time you went to bed, note any time you woke up in the middle of the night and when you went back to sleep, and when you finally woke up for good. Also, describe how you felt when you woke up. Write down any notes that you would like. Perhaps you want to describe a dream. d. Calculate the amount of time that you were up in the middle of the night. To figure this out, count the number of minutes that you were awake and add them together. e. Calculate the total time sleeping by counting the minutes between when you went to bed and when you finally woke up, and subtract any time you were up in the middle of the night. 3. Use the example provided in the PowerPoint to help students understand how to appropriately collect data and to check for understanding. 4. Remind students to bring their My Sleep Journal back to class after one week.
Ask students to fill in a practice sleep chart entry before showing them the answer.
Part 3: Sleep Chart 1. Instruct students to write their name on their Sleep Chart, and to fill in the “Day” and “Date” so they match with their My Sleep Journal. Explain to students that each “Day” begins as 12:00 am, or midnight, and ends at 11:59 pm, right 1. before the next midnight. Most of us are sleeping when a new “Day” begins. This is reflected on the Sleep Chart. Also, the Sleep Chart is split into small one hour segments, and those one hour segments are split into smaller 15 minute increments. 2. Use the example provided in the PowerPoint to help students understand how to appropriately shade in their sleep data in their Sleep Chart and to check for understanding. 3. Ask students to complete the example Sleep Chart entry before shading in their own data. If they have times that are not exactly at the :00, :15, :30, or :45 mark, they will have to make an estimate. 4. Instruct students to use a pencil to shade in the time they were sleeping for each day and date. 5. When students are finished with their Sleep Chart, instruct them to answer the questions.
How do your sleep patterns demonstrate that your body is maintaining homeostasis? Answers will vary. Although scientists do not agree on the function of sleep, there is no doubt that humans, and many other organisms, need it. Without sleep, we become tired and have trouble thinking clearly. Refer to the Day 7 “Sleep Fun Fact” in your My Sleep Journal. “Studies show that after a good night’s sleep, people tend to perform better at recognizing visual patterns and solving math puzzles. They even perform 20% better at typing!” Humans have different sleep requirements, which is dependent on age. Refer to the Day 1 “Sleep Fun Fact” of your Sleep Journal. If you are ages 5-9, you need about 11 hours of sleep a day. If you are ages 10-13, you need about 10 hours of sleep a day. Compare the appropriate value to your Sleep Chart. Are you getting enough sleep? If you are not, you will accumulate a sleep debt. Over time, your homeostatic drive to sleep will increase and become out of sync with your circadian drive to sleep. You may “make up” the sleep debt by sleeping during an unusual time of day and/or sleeping for a much longer amount of time. Sometimes people digest caffeine, but that only temporarily masks the symptoms of sleep debt.
In addition to the amount of time we sleep, we need to take into account our quality of sleep. Refer to your Sleep Chart. Does it show that you had consolidated nights of sleep, shown by entire blocks of shaded areas, or did you have disrupted sleep, shown by splotchy shaded areas? The activity that we do before we go to bed also affects our sleep. Refer to the Day 6 “Sleep Fun Fact” in your My Sleep Journal. The National Sleep Foundation suggests that calming activities before bed aid our sleep and cautions against exercising to close to bedtime. Exercise will raise our body temperature and alertness, which will make it more difficult for you to fall asleep.
In conclusion, sleep is a necessary part of life. When we do not get enough of it, or have poor quality of sleep, our sleep homeostasis will become imbalanced. In order to be healthy and efficient, we need to maintain an internal balance.
Do you think sleep is a positive feedback mechanism or a negative feedback mechanism? Why? Answers will vary.
Use the Inquiry Wheel to help students come up with independent and dependent variables.
All About You
Teacher Education Course
Introducing Michel Siffre!
Imagine how your body
would respond if you
lived in a camp in an
How would you know
when to go to sleep
without the normal cues
of night and day?
Responding to Stimuli
For each stimulus, give the expected response:
Why do you think we respond to these stimuli?
You prick your finger with a pin
Waiting for a bus in the winter
Waiting for a bus in the summer
Touch a hot iron
Hear a fire alarm in the same room
Pull your finger away
Pull your hand away
Cover your ears
Responding to Stimuli
Let’s focus on two
• Waiting for a bus in the
• Waiting for a bus in the
Why does our body
react the way it does?
• We need to maintain
homeostasis, or a state
of internal balance
Positive Feedback Mechanism
Our bodies respond to a
stimulus by increasing a
homeostasis is reached
Example: healing a cut
Negative Feedback Mechanism
Our bodies respond to a stimulus by decreasing a process in order
to maintain homeostasis
Example: blood sugar levels
Circadian rhythm controls sleep
Our circadian rhythm controls
many things in our bodies such
• body temperature
• hormone secretion
• blood pressure
• sleep cycles
Although scientists still debate
the function of sleep, it is an
activity characterized by four
• a reduction in physical activity
• display a typical body position. In
humans, we usually lay down and
close our eyes
• lowered response to external stimuli
• easily reverse the state of sleeping
Circadian and Homeostatic Drive to Sleep
The homeostatic drive to
sleep is independent
from the circadian drive
to sleep because it is no
longer affected by light, it
is affected by the history
of our sleep
Sleep is important!
“Just as nutritional status,
ambient temperature, level
of stress, blood
oxygenation, and other
variables clearly affect the
ability to learn, adequate
sleep is vital for optimal
performance in learning
Jerome M. Siegel of
the UCLA Department
of Psychiatry and
Brain Research and
the Center for Sleep
The University of Chicago
conducted a study of
volunteer students who
only received four to six
hours of sleep a night
• The students developed
higher blood pressure and
higher levels of stress
• The students also showed
some level of slowed
metabolism and insulin
resistance, which can lead to
type 2 diabetes
Activity, Part 1
Are You a Lark or an Owl? Questionnaire Scores
16-30 31-41 42-58 59-69 70-86
Sleep Journal and Sleep Chart Practice
Activity one hour
Sunday 9/23/12 Watching T.V. 11:30 pm 2:30 am 10 min 6 hours
2:40 am 6:00 am ---
How do your sleep patterns demonstrate that
your body is maintaining homeostasis?
Do you think sleep is a positive feedback
mechanism or a negative feedback