(CNN) -- The health of 33 trapped Chilean miners is authorities' top priority, as crews this week began drilling in an effort to free them.
The men have been trapped 2,300 feet underground since a rockslide cut off their exit route on August 5.
Drillers could take three to four months to reach the subterranean chamber, Chilean authorities have estimated.
Humans can survive in extreme environments, such as the 540-square-foot space the men share. But this kind of prolonged confinement with darkness, crowding and lack of sanitation can take a heavy physical toll.
Despite such adversity, humans are remarkably resilient and adaptable, health experts said.
"There's a strong instinct for human survival," said Jason Kring, assistant professor for human factors and systems at the Embry-Riddle Aeronautical University in Daytona Beach, Florida. "They happen to be forced into the situation. People will always find a way. It's the most basic instinct to find a way to survive."
Despite the challenging environment, the body will attempt to maintain the balance of oxygen, glucose and water.
"We can live without oxygen for a matter of seconds or minutes, without glucose or water for matter of days," said Lawrence Armstrong, a professor at the Human Performance Laboratory at the University of Connecticut. "Therefore, as long as they have good air supply, food and water, they could survive indefinitely in that environment."
There are other health factors, he said.
This week, a four-person team from NASA -- a doctor, a psychologist, a nutritionist and a logistics engineer -- are traveling to Chile to apply its techniques for keeping astronauts healthy in outer space to the miners.
The miners have been communicating through three holes, each about four inches in diameter.
Here are some health factors as the miners wait for what could be months buried underneath the earth.
Air and heat
Rescue workers have been pumping down compressed, cool air periodically to keep the enclosed space fresh.
The men have been trapped in the San Esteban copper and gold mine in northern Chile for nearly four weeks.
"The temperature increases, as you get deeper into the Earth," said Michael Nelson, an associate professor and chair of mining and engineering at the University of Utah. "Mines that are very deep are quite warm."
A video shot last week showed the heavily bearded men stripped to the waist. A thermometer showed the shelter to be at about 85 degrees Fahrenheit.
The heat could affect oxygen levels inside the shelter.
"If it's hot, people will breathe harder," said Dr. Andrew Wala, professor at the University of Kentucky who teaches mining and ventilation courses. "If you are resting, you breathe less oxygen and produce less carbon dioxide as exhaust. But you can calculate that being hot, they are breathing like they are working hard, you can assume there is some factor."
The lack of oxygen could cause a condition called hypoxia, in which oxygen levels dip, as carbon dioxide levels rise. To prevent breathing problems, Chilean authorities began sending oxygen to the safety shaft last week.
The miners will receive a diet approved by the nation's Department of Health, said Alejandro Pino Uribe, spokesman for the Association of Chilean Security. On Monday, they received yogurt, cereal and ham and jam sandwiches, he said. On Wednesday, they are scheduled to receive their first plates of hot food -- meat and rice.
The challenge has been in finding items that will fit down and survive the journey through the 4-inch tube that's the miners' only link to the surface. The Chilean staple of beans will not be sent to the men because of the possibility of giving them intestinal gas.
The miners can expect to get up to 2,000 calories and 5 liters of water per day.
So far, each man has lost about 22 pounds, Chile's Health Minister Jaime Manalich told reporters last week.
Being deprived of sunlight for months affects an individual's sleep cycles, mood and vitamin D intake.
"It is fairly clear that seeing sunlight has an important effect on mental health," said Dr. Alan Ducatman, professor and chair of the department of community medicine at West Virginia University, who has treated American miners.
Not having regular sunlight could cause conditions such as seasonal affective disorder, a type of depression. The lack of regular sunlight may disrupt the miner's circadian rhythm, which tells the body when to sleep or be awake. This could lead to feelings of depression, inability to function and fatigue.
"Anyone who is sleep-deprived or getting less than four to five hours a night, you begin to have difficulties, cognitively and physically on the body," said Kring, president of the Society for Human Performance in Extreme Environments, a group of researchers and practitioners who improve human safety and performance in extremely risky and challenging settings.
The miners have split into two shifts, so half can rest, while the others keep busy doing tasks, exercising or playing card games or dominoes. Stretchers that were stored in the rescue cabin are being used as beds, according to a video the men sent to the surface last week.
Three or four of the miners are showing signs of anxiety and depression, said Manalich.
"They're essentially incarcerated, not voluntarily," said Ducatman, who specializes in internal medicine and occupational health. "It's a form of incarceration in a space with a number of people that's obviously not ideal."
It's important to stimulate the miners with outside communications, Kring said.
The key is to add variety into the miners' days and have elements of surprise. In long space missions, U.S. astronauts have received surprise telephone calls from athletes or movie stars to motivate them, Kring said.
Getting telephone calls from family members and recording video messages to the world can add some variety into their day.
"The key part is to have a lot of surprise built in their day, so each day doesn't take on a monotonous schedule of waking up, doing nothing, eating and going to sleep," Kring said. "That boredom and monotony, that's the worst part of being in an isolated environment."
Over the past few days, the miners received MP3 players and small speakers with a selection of music including Mexican rancheras, Puerto Rican reggaeton and Dominican merengue.
Rescue crews have sent down playing cards and religious figures such as statuettes of saints and a crucifix, according to Chilean officials.
Chilean doctors have given the miners advice about how to keep their limited living space clean: Portions of a 1-meter-high (3.3 feet), 40-meter-long (about 130 feet) shaft are being used as a latrine. It is connected to the main cabin, which is being used for sleeping, washing and praying.
The separation is to minimize disease so that there is no mixing of human waste with food and hands.
"You have a humid, hot environment, and you have human waste accumulating at a significant pace over the course of three to four months," Kring said.
The mounting human waste will produce a stench in the already hot shelter, but even such an environment could become tolerable over time.
"For these men, they become accustomed to the sensory experience," Kring said. "If we were to go in, the smell is overwhelming. They become accustomed because the body becomes adjusted to sensory overloads."
CNN's Karl Penhaul, Esprit Smith and CNN Wires contributed to this report.