Background: Circadian rhythms are regular changes in biology or behavior that occur in a 24-hour cycle. In humans, for example, blood pressure and body temperature change in a regular way throughout each 24-hour day. Animals may eat and drink at certain times of day as well. Humans have daily cycles of behavior, too. Most people start to get sleepy after dark and have a hard time sleeping when it is light outside. In many species, including humans, circadian rhythms are controlled by a tiny structure called the biological clock . This structure is located in a gland at the base of the brain. The biological clock sends signals to the body. The signals cause regular changes in behavior and body processes. The amount of light entering the eyes helps control the biological clock. The clock causes changes that repeat every 24 hours.

Situation: Jason is an average guy who goes to work each day, lives in an apartment, exercises sometimes, goes out and drinks some weekends and just generally enjoys life. Brad, however, has chosen to live in a dark cave that has no windows. He lives in a cave that has almost no light except for some candles he lights from time to time. If he needs food or supplies, he pays someone to go get them, and they slide them through a hole that can open in the steel door at the entrance of the cave.

Question: Whose circadian rhythm will be more consistent?
Answer: Jason

Situation: Jason is an average guy who goes to work each day, lives in an apartment, exercises sometimes, goes out and drinks some weekends and just generally enjoys life. Brad, however, has chosen to live in a dark cave that has no windows. He lives in a cave that has almost no light except for some candles he lights from time to time. If he needs food or supplies, he pays someone to go get them, and they slide them through a hole that can open in the steel door at the entrance of the cave.

Question: Whose circadian rhythm will be less consistent?
Answer: Brad

Situation: Jason is an average guy who goes to work each day, lives in an apartment, exercises sometimes, goes out and drinks some weekends and just generally enjoys life. Brad, however, has chosen to live in a dark cave that has no windows. He lives in a cave that has almost no light except for some candles he lights from time to time. If he needs food or supplies, he pays someone to go get them, and they slide them through a hole that can open in the steel door at the entrance of the cave.

Question: Will Jason's biological clock be more or less out of control than Brad's?
Answer: less

Situation: Jason is an average guy who goes to work each day, lives in an apartment, exercises sometimes, goes out and drinks some weekends and just generally enjoys life. Brad, however, has chosen to live in a dark cave that has no windows. He lives in a cave that has almost no light except for some candles he lights from time to time. If he needs food or supplies, he pays someone to go get them, and they slide them through a hole that can open in the steel door at the entrance of the cave.

Question: Will Brad's biological clock be more or less out of control than Jason's?
Answer: more

Situation: Jason is an average guy who goes to work each day, lives in an apartment, exercises sometimes, goes out and drinks some weekends and just generally enjoys life. Brad, however, has chosen to live in a dark cave that has no windows. He lives in a cave that has almost no light except for some candles he lights from time to time. If he needs food or supplies, he pays someone to go get them, and they slide them through a hole that can open in the steel door at the entrance of the cave.

Question: Who is more likely to eat meals at the same time each day?
Answer: Jason

Situation: Jason is an average guy who goes to work each day, lives in an apartment, exercises sometimes, goes out and drinks some weekends and just generally enjoys life. Brad, however, has chosen to live in a dark cave that has no windows. He lives in a cave that has almost no light except for some candles he lights from time to time. If he needs food or supplies, he pays someone to go get them, and they slide them through a hole that can open in the steel door at the entrance of the cave.

Question: Who is less likely to eat meals at the same time each day?
Answer: Brad

Situation: Jason is an average guy who goes to work each day, lives in an apartment, exercises sometimes, goes out and drinks some weekends and just generally enjoys life. Brad, however, has chosen to live in a dark cave that has no windows. He lives in a cave that has almost no light except for some candles he lights from time to time. If he needs food or supplies, he pays someone to go get them, and they slide them through a hole that can open in the steel door at the entrance of the cave.

Question: Will Brad's blood pressure or Jason's blood pressure be more predictable throughout the day?
Answer: Jason's

Situation: Jason is an average guy who goes to work each day, lives in an apartment, exercises sometimes, goes out and drinks some weekends and just generally enjoys life. Brad, however, has chosen to live in a dark cave that has no windows. He lives in a cave that has almost no light except for some candles he lights from time to time. If he needs food or supplies, he pays someone to go get them, and they slide them through a hole that can open in the steel door at the entrance of the cave.

Question: Will Brad's blood pressure or Jason's blood pressure be less predictable throughout the day?
Answer: Brad's

Situation: Jason is an average guy who goes to work each day, lives in an apartment, exercises sometimes, goes out and drinks some weekends and just generally enjoys life. Brad, however, has chosen to live in a dark cave that has no windows. He lives in a cave that has almost no light except for some candles he lights from time to time. If he needs food or supplies, he pays someone to go get them, and they slide them through a hole that can open in the steel door at the entrance of the cave.

Question: Is the chance Jason feels sleepy after the sun goes down higher or lower than Brad's chance?
Answer: higher

Situation: Jason is an average guy who goes to work each day, lives in an apartment, exercises sometimes, goes out and drinks some weekends and just generally enjoys life. Brad, however, has chosen to live in a dark cave that has no windows. He lives in a cave that has almost no light except for some candles he lights from time to time. If he needs food or supplies, he pays someone to go get them, and they slide them through a hole that can open in the steel door at the entrance of the cave.

Question: Is the chance Brad feels sleepy after the sun goes down higher or lower than Jason's chance?
Answer: lower