This search for meaning is not an idle pursuit. To discover that you play a minor role in a cosmic drama that transcends your everyday self can be hugely beneficial for mental well-being. The impressive clinical effects of psychedelics recorded in studies over the past few years appear to be directly related to their ability to provoke meaningful, even spiritual experiences. The self-transcendent insights they afford have proved particularly helpful for people forced to confront their own mortality. A few years ago, when Stephen Ross and his colleagues at New York University School of Medicine gave a single dose of psilocybin (found in magic mushrooms and truffles) to twenty-nine people struggling to cope with a cancer diagnosis, the psychedelic significantly improved their quality of life and brought immediate and substantial relief from symptoms of anxiety and depression.

Their study employed a ‘crossover’ design: patients either took psilocybin in a first session followed by a placebo seven weeks later, or vice versa. Each was also given conventional psychotherapy. Six-and-a-half months after the treatment, between 60 and 80 per cent were still reporting clinically significant improvements in symptoms of anxiety and depression compared with the start of the trial. Crucially, this therapeutic effect seemed to be mediated by the spiritual insights they had while on psilocybin. Some 70 per cent of all the patients reported that, even though their trip was emotionally challenging, they rated it as among the top five most personally meaningful experiences of their entire lives.¹

One of the patients, a fifty-one-year-old woman called Erin, who had been knocked sideways by the news she only had a 50:50 chance of being alive in five years’ time as a result of ovarian cancer, described a vision that gave her a vital insight.² Under the influence of psilocybin, she saw a round dinner table:

Similarly, in a trial led by Robin Carhart-Harris at Imperial College London, reported in The Lancet Psychiatry in 2016, twelve patients with treatment-resistant major depression experienced significant, sustained reductions in their symptoms after taking just two doses of psilocybin one week apart alongside conventional psychotherapy. Again, these clinical improvements correlated with ratings of how insightful or mystical they judged the drug experience to be.^3–5

In the modern world, doctors have assumed the healing role once played by priests and shamans, but theirs is a mostly biological conception of illness that can overlook patients’ spiritual needs. Their pills are not designed to restore a sense of meaning, connectedness or purpose to people’s lives. Prozac and Valium don’t work by offering insights into one’s problems but by damping down their emotional impact, which goes a long way towards explaining why conventional antidepressants and anti-anxiety drugs like these must be taken continually for their effects to be sustained, whereas research to date suggests that just one or two doses of a psychedelic can be life-changing. Also worth bearing in mind is the fact that taking conventional antidepressant and anti-anxiety drugs is often associated with side effects such as drowsiness and sexual dysfunction, and can lead to dependence: when patients stop taking them there may be unpleasant withdrawal effects, including a sharp rebound in their original symptoms.

Age-old techniques for provoking altered states of consciousness, such as meditation, sleep deprivation, trance and hallucinogens, are well known for their ability to precipitate spiritual and emotional breakthroughs but, as I would be the first to acknowledge, they are inherently risky for some people. There is a delicate balance to be struck between laying oneself wide open to spiritually meaningful experiences and triggering a ‘psychotic episode’ in which one can lose touch with reality for days, weeks or longer, with hallucinations and possibly delusions of persecution or grandeur. Research suggests that consciousness-warping chemicals and practices allow us to dissolve rigid patterns of thought and behaviour, including drug addiction and depression, like shrugging off old clothes that have grown worn and uncomfortably tight over the years. In the process, however, they expose the naked psyche to the cold blast of painful memories and emotions. And there can be no guarantee that the new clothes the mind finds to put on will fit any better than the old ones. For a few unlucky people, in particular those vulnerable to psychosis, the fit may be more uncomfortable.

All the altered states of consciousness I explore within these pages involve what psychologists call ‘dissociation’ – a temporary disconnection from everyday reality. But for the small percentage of people who have experienced psychosis or who have a family history of bipolar disorder or schizophrenia, these states run the risk of severing this link for longer periods, perhaps permanently. Psychedelic researchers recruiting volunteers for their studies will reject applicants if they fall into these categories, as will many meditation and ayahuasca retreat centres, as I learned to my disappointment. For everyone else, a supportive environment – a calm, protected space with compassionate, experienced individuals on hand – is essential to maximize the benefits and minimize the risks associated with these extraordinary experiences, in particular allowing any difficult emotions and memories that come up to be processed safely. Just as importantly, the healing process doesn’t end when people return to ordinary consciousness. In the weeks and months that follow, the profound insights and revelations must somehow be integrated into everyday life, for example through a daily meditation practice, spending more time in nature, creative pursuits or voluntary work.

If you are wary of the idea of temporarily disengaging from reality, it’s worth remembering that this is what happens every night while you sleep. In the next chapter, I explore the idea that dreaming sleep streamlines our models of the waking world, pruning redundant synapses that have accumulated during the day’s learning experiences and helping our brains to function more efficiently. In order to do this, they must switch off almost all sensory inputs and motor outputs and suspend the mind’s reality-checking faculties.

No wonder dreams, like the delusions of psychosis, are so very convincing. But if this nocturnal housework isn’t carried out properly every night, the nervous system becomes increasingly cluttered with unnecessary connections. Like an ageing desktop computer, this means it works less efficiently, disrupting not just cognition but also homeostasis – the maintenance of a stable internal environment including essential tasks such as regulating temperature, pH and blood sugar levels. Not getting enough sleep (at least seven hours a night) is strongly associated with an increased risk of poor mental and physical health, including depression, suicidal thoughts, cancer, diabetes, heart disease and Alzheimer’s.^6,7

The effects of sleep deprivation on homeostasis are particularly striking. While research ethics committees would not allow such an experiment to be performed on humans, rats deprived of sleep for more than eleven days die as a result of a complete breakdown in their ability to maintain a stable body temperature.⁸

One of the take-home messages of this book is that you can’t reap the restorative benefits of sleep, or any other altered state of consciousness, without temporarily disconnecting from the reality checks that your senses and rational mind normally provide. According to the leading theory of how we develop and maintain our cognitive models of the world, known as ‘prediction error processing’, the streamlining that underpins the restorative powers of altered states can only occur after entire levels of the brain’s processing hierarchy have been taken offline.

The trouble starts when people mistake what they dreamed, or the visions they saw, for reality. Psychiatrists call this ‘magical thinking’. Like many people, I have come to believe that science is our best friend for judging what is and is not real, so I get uncomfortable when some folk who have experienced altered states talk earnestly about supernatural phenomena such as astral projection – the ability to shed one’s physical body and wander the cosmos – past lives and spirit guides. Like the contents of a dream in the moments after awakening, these things can seem all too real. Not even scientists are immune to magical thinking. Shortly before I travelled to Peru, I interviewed a highly respected researcher who has investigated the potential clinical benefits of drinking ayahuasca and personally taken part in many ceremonies. The changes in his worldview apparently wrought by the medicine were startling:

The paradox of altered states of consciousness is that they restore openness, flexibility and meaning to our lives by temporarily messing with our reality-checking faculties. What fascinates me is that in the process, they reveal so much about how the brain strives to make sense of the world. The more we have learned about altered states, the more light they have shone on ordinary waking consciousness, exposing the hidden hand of belief, expectation and brain chemistry in everything we experience.

Optical illusions demonstrate that, as the British psychologist Richard Gregory observed in 1980, perceptions are like scientific hypotheses.⁹ They are our best guesses based on the limited evidence available to us. Because sensory stimuli are inherently ambiguous, the brain must weigh the rough-and-ready ‘rules of thumb’ it has learned (its expectations based on past experience) against the likelihood that the available sense data is reliable. The result of this subconscious calculation is conscious perception. The classic optical illusions shown in Figure 1 are caused when the brain invests too much confidence in predictions based on its rules of thumb, giving insufficient credence to raw sensory information. In the Café Wall (A), Kanisza Square (B) and Poggendorff (C) illusions, the context fools the brain into applying rules that lead us to see, respectively, parallel lines as converging or diverging; a complete square; and two segments of a diagonal line as offset rather than aligned. In the Rubin’s Vase illusion (D) the brain vacillates between two alternative interpretations: a vase or two faces.

Clearly there’s more to conscious perception than a stream of predictions flowing in one direction and a stream of prediction errors coming back in the other. Without some way to filter or vet the barrage of prediction errors, the mind would be quickly overwhelmed or easily misled. What altered states reveal is the importance of the brain’s chemical tuning systems. Karl Friston, a neuroscientist based at University College London who helped pioneer the theory of prediction error processing, told me how these might work. For a start, he said, our brains don’t need to pay any attention to sense data that exactly match their predictions: