neuroscience - Can parts of a human brain be asleep independently of each other, or vary in the times required for them to fall asleep?

I know that some birds and marine animals can continue complicated activity (swimming, flying?) while one hemisphere of their brain is asleep.

I'm interested if human brain has some parts of it that can be asleep while others are awake? In other words, can a human brain be only partially asleep while experiencing insomnia or similar sleep disturbances?

If human brain can have different parts "sleeping" independently of each other, is it possible that the times to "fall asleep" vary between these different parts of the brain?

I would appreciate research articles on the topic or just the names of brain regions that may exhibit behavior described above.

Update: I've taken a look at R&K "principles and practice of sleep medicine"' and it mentions the following parts as involved in sleep:

Medulla, preoptic area, hypothalamus, thalamus, entire neocortex involved in NREM.

Neurotransmitter systems: histaminergic, orexinergic, serotonergic, noradrinergic

Sleep factors: adenosine, interleukin-1 and other cytokines, prostaglandin D2, growth hormone releasing hormone, nitric oxide, all promote sleep in or around preoptic area.

This makes me hypothesize that drugs that modify the effects of these systems (ex- caffeine affecting adenosine) could result in sleep- related disturbances in these systems, potentially causing them to fall asleep later that usual. But I'm looking for more info to fihre out if this is true

Answer

Reading K&R "Principles and practice of sleep medicine" 4th edition, on page 15, under sleep onset I've noticed the following paragraph:

Is "falling asleep" a unitary event? Our observations suggest that it is not. Different functions, such as sensory awareness, memory, self-consciousness, continuity of logical thought, latency of response to a stimulus and alterations in the pattern of brain potentials all go in parallel in a general way, but there are exceptions to every rule.

The book continues to mention that it is possible to drift in and out of sleep for a few seconds.

I've became aware of experiments on split brain patients (severed corpus colossum) which revealed that two brain hemispheres have their own conscious awareness. Some experiments even paralyzed one hemisphere to see how the other one responds. As such, it appears that a human can function with only one hemisphere active at a time.