These vary so greatly that it is difficult to venture any general comments. On one extreme there exist monks and nuns whose whole lives are ordered around meditation; on the other hand, one-minute meditations are not out of the question.

Twenty or thirty minutes is broadly accepted as being a typical duration. Experienced meditators often find their sessions growing in length of their own accord. Observing the advice and instructions of one's spiritual teacher is generally held to be most beneficial.

Many traditions stress regular practice. Accordingly, many meditators experience guilt or frustration upon failing to do so. Possible responses range from perseverance to acceptance.

The purposes for which people meditate vary almost as widely as practices. Meditation may serve simply as a means of relaxation from a busy daily routine; as a technique for cultivating mental discipline; or as a means of gaining insight into the nature of reality, or of communing with one's God. Many report improved concentration, awareness, self-discipline and equanimity through meditation.

Many authorities avoid emphasizing the effects of meditation — sometimes out of modesty, sometimes for fear that the expectation of results might interfere with one's meditation. For theists, the effects of meditation are considered a gift of God or from the Holy Spirit/Ghost, and not something that is "achieved" by the meditator alone, just as some say that a person will not convert to Christianity without the influence of the Holy Spirit/Ghost's presence.

At the same time, many effects (or perhaps side-effects) have been experienced during, or claimed for, various types of meditation. These include:
Greater faith in, or understanding of, one's religion or beliefs
An increase in patience, compassion, and other virtues and morals or the understanding of them
Feelings of calm or peace, and/or moments of great joy
Consciousness of sin, temptation, and remorse, and a spirit of contrition.
Sensitivity to certain forms of lighting, such as fluorescent lights or computer screens, and sometimes heightened sense-perception.
Surfacing of buried memories, possibly including memories of previous lives and those of others, or that of those to come
Experience of spiritual phenomena such as kundalini, extra-sensory perception, or visions of deities, saints, demons, etc.
"Miraculous" abilities such as levitation (cf. yogic flying)
Psychotic episodes (see medical section below)

Some traditions acknowledge that many types of experiences and effects are possible, but instruct the meditator to keep in mind the spiritual purpose of the meditation, and not be distracted by lesser concerns. For example, Mahayana Buddhists are urged to meditate for the sake of "full and perfect enlightenment for all sentient beings" (the bodhisattva vow). Some, as in certain sects of Christianity, say that these things are possible, but are only to be supported if they are to the glory of God.

In the recent years there has been a growing interest within the medical community to study the physiological effects of meditation (Venkatesh et al., 1997; Peng et al., 1999; Lazar et al., 2000; Carlson et.al, 2001). Many concepts of meditation have been applied to clinical settings in order to measure its effect on somatic motor function as well as cardiovascular and respiratory function. Also the hermeneutic and phenomenological aspects of meditation are areas of growing interest. Meditation has entered the mainstream of health care as a method of stress and pain reduction. For example, in an early study in 1972, transcendental meditation was shown to affect the human metabolism by lowering the biochemical byproducts of stress, such as lactate, decreasing heart rate and blood pressure and inducing favorable brain waves. (Scientific American 226: 84-90 (1972)). In 1976, the Australian psychiatrist Ainslie Meares, reported in the Medical Journal of Australia, the regression of cancer following intensive meditation. Meares would go on to write a number of books, including his best-seller Relief without Drugs.

As a method of stress reduction, meditation is often used in hospitals in cases of chronic or terminal illness to reduce complications associated with increased stress including a depressed immune system. There is a growing consensus in the medical community that mental factors such as stress significantly contribute to a lack of physical health, and there is a growing movement in mainstream science to fund research in this area (e.g. the establishment by the NIH in the U.S. of 5 research centers to research the mind-body aspects of disease.) Dr. James Austin, a neurophysiologist at the University of Colorado, reported that Zen meditation rewires the circuitry of the brain in his landmark book Zen and the Brain (Austin, 1999). This has been confirmed using functional MRI imaging which examine the electrical activity of the brain.

Dr. Herbert Benson of the Mind-Body Medical Institute, which is affiliated with Harvard and several Boston hospitals, reports that meditation induces a host of biochemical and physical changes in the body collectively referred to as the "relaxation response" (Lazar et.al, 2003). The relaxation response includes changes in metabolism, heart rate, respiration, blood pressure and brain chemistry. Benson and his team have also done clinical studies at Buddhist monasteries in the Himalayan Mountains.

Other studies within this field include the research of Jon Kabat-Zinn and his colleagues at the University of Massachusetts who have studied the effects of mindfulness meditation on stress (Kabat-Zinn et.al, 1985; Davidson et.al, 2003).

Mindfulness meditation and related techniques are intended to train attention for the sake of provoking insight. Think of it as the opposite of attention deficit disorder. A wider, more flexible attention span makes it easier to be aware of a situation, easier to be objective in emotionally or morally difficult situations, and easier to achieve a state of responsive, creative awareness or "flow".

One theory, presented by Daniel Goleman & Tara Bennett-Goleman (2001), suggests that meditation works because of the relationship between the amygdala and the prefrontal cortex. In very simple terms, the amygdala is the part of the brain that decides if we should get angry or anxious (among other things), and the pre-frontal cortex is the part that makes us stop and think about things (it is also known as the inhibitory centre).

So, the prefrontal cortex is very good at analyzing and planning, but it takes a long time to make decisions. The amygdala, on the other hand, is simpler (and older in evolutionary terms). It makes rapid judgments about a situation and has a powerful effect on our emotions and behaviour, linked to survival needs. For example, if a human sees a lion leaping out at them, the amygdala will trigger a fight or flight response long before the prefrontal cortex responds.

But in making snap judgments, our amygdalas are prone to error, such as seeing danger where there is none. This is particularly true in contemporary society where social conflicts are far more common than encounters with predators, and a basically harmless but emotionally charged situation can trigger uncontrollable fear or anger — leading to conflict, anxiety, and stress.

Because there is roughly a quarter of a second gap between the time an event occurs and the time it takes the amygdala to react, a skilled meditator may be able to intervene before a fight or flight response takes over, and perhaps even redirect it into more constructive or positive feelings.

The different roles of the amygdala and prefrontal cortex can be easily observed under the influence of various drugs. Alcohol depresses the brain generally, but the sophisticated prefrontal cortex is more affected than less complex areas, resulting in lowered inhibitions, decreased attention span, and increased influence of emotions over behaviour. Likewise, the controversial drug ritalin has the opposite effect, because it stimulates activity in the prefrontal cortex.

Some studies of meditation have linked the practice to increased activity in the left prefrontal cortex, which is associated with concentration, planning, meta-cognition (thinking about thinking), and positive affect (good feelings). There are similar studies linking depression and anxiety with decreased activity in the same region, and/or with dominant activity in the right prefrontal cortex. Meditation increases activity in the left prefrontal cortex, and the changes are stable over time — even if you stop meditating for a while, the effect lingers.

Electroencephalographs (EEG) recordings of skilled meditators showed gamma wave activity that gradually expanded across the brain during meditation. Gamma waves indicate synchrony between sections of the brain. These meditators had 10 to 40 years of training in Buddhist-based mental training. EEGs done on meditators who had received recent training turned up considerably less synchrony.

The experienced meditators also showed increased gamma activity while at rest and not meditating. The results of the study do not make clear whether meditation training creates this activity or if individuals with high gamma activity are attracted to meditation. (Antoine Lutz & Richard J. Davidson, 2004).

Meditation also affects brain wave production as measured by an EEG machine. While the brain at a waking state is primarily in the Beta range of frequencies (14 - 21 cycles per second), while under meditation the brain tends to slow down the Alpha range (7 - 14 cycles per second). One of the first Americans to study the effects of meditation on brainwave production was José Silva who founded the Silva Method. Silva theorized that meditation, in addition to stress relief, could also be used for enhancing creativity and developing intuition.