Our senses are not as sensitive during sleep compared to wake. There is an arousal threshold during sleep that only when it is overcome will it wake a person up. My question is how is this arousal threshold lowered as sleep progresses? And how it is raised again during the latter stages of sleep?
At night several changes occur in the body due to absence of sunlight and other surrounding changes. The biological clock inside the body performs certain function in a healthy body whenever changes are sensed.
There are many receptors in our body located on cell membrane. They are made up of proteins and their function is to receive information from neurotransmitters. This information will be sent to specific part of the brain. If we take sensory information then it is Parietal lobe. This lobe receives the sensory information from all over the body and responds to it when body is active. During sleep, the senses that go through the thalamus, are shut down by gating either in the Nucleus Reticularis Thalami (NRT) or in the Thalamus itself. GABAergic inhibition of the thalamus, most likely deriving from the NRT is probably a part of the reason that sensory stimuli doesn't penetrate during sleep.
This is also influenced by release of hormones in the body whose level when rises in the blood causes certain changes in the concentration at some regions such as receptors which gets blocked temporarily and stops signaling the brain. There is a study on how muscle relaxes when we sleep can be found here Muscle sleep. So it is all the neurotransmitters job at the receptors which stops signals going into brain but internal to the brain it will be active controlling other routine works which it has to do when we are asleep.
The main thing to note here is the 5 different stages of sleep such as non-REM and REM sleep. During those stages several changes take place in the body. The below website gives an excellent insight into different changes happening inside the body when body takes rest at night.
Please refer to "What happens to your body while you are asleep".
When you're awake; the sensations around you as well as your mental will, keep your mind active. How the air smells and what needs to be done all contribute to your day. In order to sleep your brain must weaken these receptors significantly, though not enough that if we heard something too loud or unfamiliar (such as a predator), we overlap those boundaries that keep us resting and wake. Now of course i am talking about the middle of the night. As morning arrives the amount of stimuli needed to overcome the wall that is sleep is diminished, allowing something such as the crowing of a rooster to wake us up.
Another lead are sleep spindles, closely related to the RTN as mentioned before. Basically the thalamus enters a state of high-frequency firing to a low-frequency synchronous one, thereby taking the cortex with it. Sleep spindles are slow-wave potentials as apparent on the EEG that block out the normal flow of peripheral sensory information from thalamus to cortex. Basically this leads to a loss of consciousness as one is unaware of its environment. A similar thing occurs during absence epileptic seizures, where patients temporarily loose consciousness as marked by paroxysmal slow-wave EEG activity interrupting the high-frequency EEG associated with consciousness Kostopaulus (2000). Hope this helps.