The sleep cycle
Stage 1 – Falling asleepThe transition from wakefulness to sleep. Only a few minutes of our total sleep time is spent in this stage and we are very easily awakened.
Stage 2 – Sound sleepWe spend about 40% to 60% of our sleep time in this stage. Clear physiological changes are noticeable: breathing becomes slower, our heart frequency and blood pressure drop and our muscles relax. As our metabolism slows, our body temperature falls by about a 10th.
These stages make up our deep sleep and it should constitute between 15% and 20% of our total sleep time. It is difficult to wake us up, requiring a noise level of about 70 dB (equivalent to the sound of a small motorbike) to wake us up. The growth hormone is excreted and it is probably now that most of the brain’s recuperation occurs. The muscles are extremely relaxed, we hardly move at all, but some people, mainly men, start to snore. It has been a couple of hours since we fell asleep and we change gear up to stage 2 again. We start to move again and perhaps wake up for a few seconds. The struggle between 'normal sleep' and the next stage, dream sleep, begins now. Deep sleep is deepest at the beginning when most of the recuperation occurs.
Rapid Eye Movement sleep makes up the classic dream sleep. It should constitute 20-25% of our total sleep time. We now know that we also dream during the other stages, mainly during stage 2. The recurring REM sleep is however important for our sleep to achieve a satisfactory quality. It has particular significance as impressions and knowledge from our short term memory are transferred to our long term memory during this period. Our muscles relax even more and even if we wanted to we couldn’t get up now. If that block wasn’t there, we could go and live out our dreams, which would be dangerous. Breathing, heart frequency and blood pressure increase. We arrive at the same level that we have when we are awake, sometimes higher. The change comes suddenly and can mean an increased risk for heart attacks, which is related to what we dream about.
