How are our senses dimmed during 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".