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From what I remember from my neuroscience classes, TL;DR there are different networks that promote sleep vs wakefulness. Sleep involves firing of specific networks that are so in sync they generate brain waves that can be measured from the outside of your skull. Think delta waves. Alcohol has many non-specific targets in the brain that fuck up this rhythmic firing that keep you from entering REM and deep sleep.

The hypothalamus, a peanut-sized structure deep inside the brain, contains groups of nerve cells that act as control centers affecting sleep and wakefulness. Within the hypothalamus is the suprachiasmatic nucleus (SCN)—clusters of thousands of cells that receive information about light exposure directly from the eyes and control your behavioral rhythm. Some people with damage to the SCN sleep erratically throughout the day because they are not able to match their sleep/wake cycle (circadian rhythms) with the light-dark cycle. Most blind people maintain some ability to sense light to help them regulate their sleep/wake cycle.

The brainstem, which is made up of structures called the pons, medulla, and midbrain, controls the transitions between wake and sleep. Sleep-promoting cells within the hypothalamus and the brain stem produce a brain chemical called GABA, reduces activity in the hypothalamus and the brainstem. The brainstem (especially the pons and medulla) also plays a special role in REM sleep. It sends signals to relax muscles essential for body posture and limb movements, so that we don’t act out our dreams.

The thalamus sends and receives information from the senses to the cerebral cortex. The cerebral cortex is the covering of the brain that has many functions, including interpreting and processing short- and long-term memory. During most stages of sleep, the thalamus becomes quiet, letting you tune out the external world. But during REM sleep, the thalamus is active, sending the cortex images, sounds, and other sensations that fill our dreams.

The pineal gland, located within the brain’s two hemispheres, receives signals from the SCN and increases production of the hormone melatonin, which helps put you to sleep once the lights go down. Scientists believe that peaks and valleys of melatonin over time are important for matching the body’s circadian rhythm to the external cycle of light and darkness.

The basal forebrain, near the front and bottom of the brain, also promotes sleep and wakefulness, while part of the midbrain acts as a system to help us stay alert during the day. Release of a chemical called adenosine from cells helps make you feel sleepy. Caffeine counteracts sleepiness by blocking the actions of adenosine.

The amygdala, an almond-shaped structure involved in processing emotions, becomes increasingly active during REM sleep.

Alcohol has non specific targets in the brain. It reduces excitatory action through NMDA ligand gated ion channels. It enhances inhibitory action through GABA ligand gated ion channels. Fucking with those ion channels changes ions entering neurons which changes electrical signals that eventually propogate down the axon. Electrical signals in a nerve axon lead to the axon terminals releasing neurotransmitters to the next cell in the chain of the entire network. This fucks up the overall network/brain waves and fucks with your ability to enter deep sleep.

You can fall asleep faster with ethanol on board, but quality of sleep will be poor.