During sleep the eyes are closed, so that the brain to some degree becomes isolated from the outside world. Moreover all signals from the senses (except olfaction) must pass through the thalamus before they reach the brain cortex, and during sleep thalamic activity is suppressed. This means that the brain mainly works with signals from itself. A well-known phenomenon in dynamical physical systems where the level of input and output from the system is low is that oscillation makes spontaneous resonance patterns to occur. Hence, dreams may be the simple consequence of neural oscillation.