Long before people hopped on the organic bandwagon to keep pesticides out of their produce or sought out BPA-free containers, cleansing was an established health- and beauty-bolstering practice around the globe.Hippocrates, known as the father of modern medicine, recommended periodic fasting to cleanse the digestive tract and rest the digestive system back in the fourth century B.C. And Paracelsus, a well-regarded 16th century physician, called fasting the greatest remedy—the physician within.But even beyond its use as a healing tool for the body, cleansing was a practice deeply rooted in religion and culture, thought to purify the mind and spirit.The fasts practiced years ago prepared the body, mind and spirit for religious worship, celebration and mourning in both Western and Eastern tradition. “For centuries, people have taken advantage of cleansing as an integral part of their cultural and religious tradition,” says Ann Louise Gittleman, Ph.D., author of “The Fast Track Detox Diet.” “They did it to enhance their spiritual and mental power. They did it for preparation for ceremony and for atonement.”Even today fasting remains an important element of religious observance, from Lent in the Christian faith to the Muslim observance of Ramadan to Yom Kippur in Judaism. In the past, Ayurvedic practitioners and yogis promoted cleansing in India, Native Americans built detoxifying sweat lodges and Europeans underwent doctor-prescribed health treatments like colonic irrigation, water treatments and sessions in the sauna.MORE: Guide to Alternative MedicineBut the times, they are a-changin’. Thanks to the post-Industrial Revolution, environmental pollution, chemicals, processed and pesticide-laden foods, as well as the stress of everyday life, have increased—and so has the burden on our bodies. According to the National Resources Defense Council, more than 80,000 chemicals used in the United States have not been fully assessed on how they affect human and environmental health. “We are finding that over two billion pounds of toxic chemicals are released into the environment every year,” says Gittleman. For some, that’s all the reason they need to make use of an ancient practice.But this isn’t your Grandma’s cleanse anymore. “Cleansing is no longer a whole body experience,” says clinical nutritionist Inna Topiler. “It’s more, eat this, don’t eat that.” Where fasting was once a time of physical and emotional reprieve, we now integrate juice fasts and colon cleanses into our busy lives like we squeeze hair and nail appointments into our overbooked schedules.A tag line of the Blueprint Cleanse, “We think, you drink,” suggests that modern cleansing doesn’t take any more thought than cracking open a bottle of juice and gulping it down. It’s become so easy, in fact, that most of us pay no mind to our body’s need for rest as we cleanse. “I tell my patients all the time, you don’t want to schedule too much during a cleanse,” says Topiler. “Don’t do as much physically, get plenty of sleep and take care of your body.”Busy as we are, we don’t always listen. “What is different today is that people think they don’t have time for retreats, actually don’t have time or cannot afford it,” says Alejandro Junger, M.D., who is board certified in internal medicine and cardiology and the author of “Clean: The Revolutionary Program to Restore the Body’s Natural Ability to Heal Itself.” “[Juice fasts] are too intense for people who are living their regular life, going to work and taking the kids to school.”QUIZ: Sleep Quality QuizEven though there’s a lack of scientific evidence (a 2000 study in Alternative Therapies in Health and Medicine that found a seven-day detoxification program significantly improved symptoms of poor health is one of the few documented examples), many experts back the positive effects of cleansing. Supporters tout that cleansing can boost your immune system, energy levels and mental focus, eliminate excess pounds and make you more aware of the foods you put in your body post-cleanse.But they also warn of complications that can come with cleansing an already burdened body. “If your liver is not up to task and prepared for the cleansing process you can be doing your body much more harm than good,” says Gittleman. “You can’t just do any old fast anymore, because you could be harming yourself much more so than if you took the proper nutrients to supplement and support the detox pathways.” An August 2011 study published in the Journal of Family Practice showed the dangers of colon cleansing in particular, from cramping and bloating to nausea and renal failure.MORE: New Study Pooh-Poohs Colon CleansingWith a flood of fast-fix juice cleanses now easily available—from Blueprint Cleanse to Cooler Cleanse to Organic Avenue—there is a real risk that even the best-laid detox plans could be harmful if you don’t take the time to rest during the process. So if you’re considering test-driving a juice cleanse, take the time to not only chill your juice, but to also chill out.MORE: Can a Cleanse Lead to Healthier Skin?