From a scientific point of view we can make no distinction between the man who eats little and sees heaven and the man who drinks much and sees snakes - Bertrand Russell
The temporal lobe of the brain is behind your ears and regulates memory, awareness of our surroundings and raw emotion. Canadian scientist Dr Michael Persinger has developed a "God-Helmet" that uses electromagnetism to stimulate the temporal lobes.
80% of Persinger's experimental subjects report that an artificial magnetic field focused on those brain areas gives them a feeling of 'not being alone'. Some of them describe it as a religious sensation.
there are certain circuits within the temporal lobes which have been selectively activated. Their activity ...makes them more prone to religious belief - V.S. Ramachandran
it is too simplistic to see religion as either spiritually inspired or the result of social conditioning ... it seems whether God exists or not, the way our brains have developed, we will go on believing - V.S. Ramachandran
an attack of temporal lobe epilepsy, perhaps ending in a convulsion - D. Landsbrough
in order to keep me from becoming conceited, I was given a thorn in my flesh, a messenger of Satan, to torment me - 2 Corinthians 12: 7
As you know, it was because of an illness that I first preached the gospel to you - Galatians 4: 13
The story of Pentecost contains an interesting detail: when the followers of Jesus start speaking in tongues, the crowd in the street assume they are drunk. Peter takes a stand and argues that they are NOT drunk, pointing out it is only 9am. This shows that the authors of the Bible knew that religious experience could look to outsiders very like drunkenness. Peter's response is to appeal to Occam's Razor (although he doesn't call it that, of course): how likely is it that so many people would be identically drunk so early in the morning?
To say [God] hath spoken to him in a dream is no more than to say he dreamed that God spake to him - Thomas Hobbes
Mass hysteria certainly explains some religious experiences - the "Dancing Plague" of 1518 is a good example. Hundreds of people started dancing uncontrollably for weeks on end. The dancers experienced religious ecstasy and those who watched felt compelled to join in. Some of the dancers died from heart attacks or exhaustion. The phenomenon was known as "Saint Vitus' Dance" and there were other outbreaks during the Middle Ages.
Some historians link the Dancing Plague to ergot poisoning, but mass hysteria would explain why it spread to affect onlookers.
do not believe every spirit, but test the spirits to see whether they are from God, for many false prophets have gone out into the world - 1 John 4: 1
Naturalistic interpretations are economical - they explain things without leaving even more questions to answer. This is called Occam's Razor, the principle that explanations should not be multiplied unnecessarily. If a religious experience can be completely explained by something we know exists (like drugs or mental illness), then this is better than explaining it with something we don't know exists (like God).
Naturalistic interpretations alert us to the fact that so-called 'religious experiences' are often a cry for help. The experient needs medical treatment or counseling. If Peter Sutcliffe's bizarre beliefs about God talking to him had been recognised as schizophrenia, he would have been treated before he killed anyone.
Naturalistic interpretations aren't really economical at all. The idea that a healthy person who doesn't take drugs and who has no history of mental illness would suddenly develop hallucinations and delusions about God definitely leaves us with more questions to answer - especially if this person is happy and healthy in every other way, showing no other negative symptoms.
Some religious experiences are inauthentic and should be recognised as a cry for help. Religions admit this and don't treat every religious experience as authentic. But just because some things that seem to be religious experiences are really health problems, it doesn't mean they all are.