What actually is a dream? In oneirology, there are different ways we can define ‘dream’. They may be different to your definition of ‘dream’, or the definition of ‘dream’ in other contexts or cultures. There are two main ways oneirology defines dreams.
The first defines dreaming as all conscious experiences during sleep. If we take this definition, then we can look at dreams from across the entire sleep spectrum, and try and get a picture of what our minds are doing at different times of night, and in different stages of sleep. We can ask questions like, “Are dreams different in REM sleep to non-REM sleep?”, and “Are dreams different in the first hour of sleep compared to the last hour?” These kinds of questions help us understand both sleep and dreaming.
This definition also allows us to consider any kind of mental (meaning “of the mind” in this context) experience that occurs during sleep to be a dream. It doesn’t have to conform to any predefined set of characteristics in order to be considered a dream. For example, sometimes people have strange images appear before their eyes in the few minutes that it takes for them to fall asleep. This would count as a dream, because it happens during the first stage of sleep. Likewise, a simple thought like “I must remember to book a dentist appointment tomorrow” would count as a dream if it happened during sleep (and these kind of thought-like dreams do occur, primarily in non-REM sleep). In this way, we can consider the full spectrum of mental content during sleep, and try to really understand what our minds are doing at night.
The second definition, conversely, takes a narrower definition of dreaming. It says that dreams are only dreams if they involve an imagined (or hallucinated), simulated world, with the dreamer being present in that world. In other words, we’re only dreaming when we’re immersed in a virtual reality of our own imaginations. For example, you might dream that you’re late for work, on a spaceship, or in your childhood home. There is an imagined world that you occupy. Any other conscious experiences that we have during dreaming, such as thinking about booking a dentist appointment, would not be called a dream, even if they occurred during sleep. This kind of dreaming has been labelled an “Immersive Spatiotemporal Hallucination” (Windt, 2010).
With this definition, we can see how dream-like experiences can also occur outside of sleep. We could have an immersive experience in a hallucinatory world during a particularly engaging day-dream, for example, or during a psychedelic trip, or in the form of an out-of-body experience, or any form of hallucinating – of which there are many (Sacks, 2013). It aligns dreaming with other forms of hallucinatory experiences rather than sleep per se. This is important because this kind of experience is not always unique to sleep, and if we only consider dream-like experiences had during sleep, we miss out on a whole number of related experiences that may also help us to understand sleep, dreaming, and hallucinating.
Since there are upsides and downsides to both definitions, perhaps a combination of the two may be able to give us the best of both worlds, such as:
Sacks, O. (2013). Hallucinations. Picador.
Windt, J. M. (2010). The immersive spatiotemporal hallucination model of dreaming. Phenomenology and the Cognitive Sciences, 9(2), 295–316.
 The word ‘conscious’ here means that there is a subjective, qualitative feel to the experience, rather than psychoanalytical meaning of ‘conscious’ being in contrast to ‘unconscious’.