-The Role of Genes and Nuerons, Neurotransmitters
-Chemical signals causing sleep, how we  fall asleep :    

Clusters of sleep-promoting neurons in many parts of the brain become more active as we get ready for bed.  
Nerve-signaling chemicals called neurotransmitters can “switch off” or dampen the activity of 
cells that signal arousal or relaxation.  GABA is associated with sleep, muscle relaxation, and sedation. 
Norepinephrine and orexin (also called hypocretin) keep some parts of the brain active while we are awake.  
Other neurotransmitters that shape sleep and wakefulness include acetylcholine, histamine, adrenaline, cortisol, and serotonin.

First stage of sleep: brain waves slow down, shifting to a form known as theta-band activity,
 but are still punctuated by brief bursts of alpha activity. 
These hiccups give you the sense that you're still awake, said Scott Campbell, 
director of the Laboratory of Human Chronobiology at Weill Cornell Medical College, 
citing a landmark sleep study performed in the 1960s. 
"Investigators asked subjects aroused out of various stages of sleep whether they considered 
themselves asleep. Only about 10 percent of those aroused from stage 1 said that they had been asleep."

Think of what happens when you doze off while watching a movie: Y
ou remember bits and pieces of scenes for quite a while before conking out completely. 
Those excerpts — picked up during the short bursts of alpha-band activity in your brain
give you a sense that you're awake,  though you're actually well on your way to dreamland.

Next, your brain moves on to stage 2, the start of "true" non-REM (rapid eye movement) sleep, when those bursts 
of alpha activity die down. All neuroscientists agree that this stage is sleep, though you still might not know it. 
In that same 1960s study of sleep arousal, "about 60 percent believed that they had been asleep when aroused out of stage 2," 
Campbell told Life's Little Mysteries. The other 40 percent would tell you they hadn't fallen asleep yet.

This makes sense in light of a 2010 study by Taiwanese neuroscientists. 
They demonstrated that sleep stage 2 is associated with further reductions in the perception 
of external stimuli. Though we're asleep, we might still hear a word or two of dialogue 
from that movie; it gradually fades away.

Next, we enter slow wave sleep (also known as deep sleep, delta-band activity, or stages 3 and 4)
and finally, experience REM sleep — the stage when we dream. While stages 1 and 2 are difficult to perceive, 
90 percent of people recognize themselves as having definitely fallen asleep when aroused after entering stages 3 or 4. 
That means we've completed the transition. 
From then on, we spend the rest of the night cycling between non-REM stages 2, 3 and 4, and REM sleep.

Stage 3 sleep is also known as deep sleep, and it is harder to wake someone up if they are in this phase. 
Muscle tone, pulse, and breathing rate decrease in N3 sleep as the body relaxes even further. 
The brain activity during this period has an identifiable pattern of what are known as delta waves.

The rhythmic electric fields generated by the brain during deep sleep and other periods of intensely coordinated neural activity 
could amplify and synchronize actions along the same neural networks that initially created those fields.
The neuron is the basic working unit of the brain, a specialized cell designed to transmit information 
to other nerve cells, muscle, or gland cells. Neurons are cells within the nervous system 
that transmit information to other nerve cells, muscle, or gland cells. 
Neurons are the cells that make up the brain and the nervous system. 
They are the fundamental units that send and receive signals which allow 
us to move our muscles, feel the external world, think, form memories 
and much more.

Neurotransmitters are often referred to as the body's chemical messengers. 
They are the molecules used by the nervous system to transmit messages 
between neurons, or from neurons to muscles. 
Communication between two neurons happens in the synaptic cleft (the small gap between the synapses of neurons). 
Here, electrical signals that have travelled along the axon are briefly converted into chemical ones 
through the release of neurotransmitters, causing a specific response in the receiving neuron