Historically this time of wakefulness between the two sleep segments was considered a special and even sacred time. Laborers tired from working the fields would have the energy to have sexual relations after their first period of sleep, people engaged in spiritual practices such as prayer, reflection and meditation, and others used the quiet time to read and write. Dreams were more available to be remembered during that time than upon awakening in the morning.” – Source

“The historian Robert Ekirch summarized in his book At Day’s Close: Night in Times Past, the abundant historical evidence that humans previously slept in two separated segments. This custom was common knowledge and there are many references to it. Here is an interesting passage from that article;

He unearthed ”more than 500 references to a segmented sleeping pattern – in diaries, court records, medical books, and literature, from Homer’s Odyssey to an anthropological account of modern tribes in Nigeria.

During this waking period, people were quite active. They often got up, went to the toilet or smoked tobacco and some even visited neighbors. Most people stayed in bed, read, wrote and often prayed. Countless prayer manuals from the late 15th Century offered special prayers for the hours in between sleep.

And these hours weren’t entirely solitary – people often chatted to bed-fellows or had sex.”

A doctor’s manual from 16th Century France even advised couples that the best time to conceive was not at the end of a long day’s labor but “after the first sleep”, when “they have more enjoyment” and “do it better”. Ekirch found that references to the first and second sleep started to disappear during the late 17th Century. This started among the urban upper classes in northern Europe and over the course of the next 200 years filtered down to the rest of Western society. By the 1920s the idea of a first and second sleep had receded entirely from our social consciousness. He attributes the initial shift to improvements in street lighting, domestic lighting and a surge in coffee houses – which were sometimes open all night. As the night became a place for legitimate activity and as that activity increased, the length of time people could dedicate to rest dwindled.” – Source

Some people might think they can simply wake up and go back to sleep, but this does not yield good results. Without at least an hour and a half-awake between segments, the body treats the two segments as one interrupted monophasic sleep the hormonal surge and sleep regulation do not happen. Jessica Gamble suggests that most people will spend two hours awake between segments, some people may be awake for more or less. Perfect timing doesn’t matter for this kind of schedule, not as much as other more hardcore schedules, so don’t worry about one or two nights that don’t allow you to sleep as you planned. But as always, consistency is healthy. Early to bed, Early to rise, with a few hours awake between, makes a man healthy, wealthy, and wise.

It is hypothesized that this segmentation is the origin of the ‘graveyard’ hours most humans experience – a sudden and strong wave of tiredness that hits around 3-4.30 AM. It is proposed that this wave of tiredness is caused by a surge of progesterone which causes sleepiness. This is a sign of healthy sleep and supports the segmented sleep theory, as a segmented sleeper would need a physiological cue to take their second segment of sleep after being awake for a few hours.

SEGMENTED SLEEP AND SLEEP ARCHITECTURE

There are many changes to the sleep architecture when you adapt to segmented sleeping. Normally SWS will be spread out across the night, but it will accumulate in the first three hours of your sleep, usually resulting in higher delta band activity and deeper sleep. Normally choline will fluctuate throughout the night, but when you wake up your brain will start to refill its banks of choline and when you sleep your second segment you will have cholinergic REM sleep, which means you will have more dense REM and will remember your dreams more easily, and more likely dream lucidly.

Light sleep will usually decrease, and your overall SWS and REM sleep will increase, so some people may find they sleep less, others may find they sleep better.

SEGMENTED SLEEP AND HORMONES

There is evidence that the prolactin hormone surge associated with regular and stable night sleep segmentation can lead to improved sleep architecture, increasing restorative SWS in the first sleep, and increasing REM in the second sleep.

This prolactin surge in the first segment of the night can be regulatory of the entire day’s hormonal secretions! Having high peak prolactin in the night, and repartitioning SWS into the first three hours of the night will mean there is less prolactin secreted throughout the day.

For the biology nerds:

Prolactin enhances the secretion of dopamine and HGH, which enhances the delta wave SWS. Prolactin also down-regulates sex hormones, so when the prolactin is high and blocking other hormones there effectively is a sex-hormone rebound when the prolactin secretion stops when you wake. When the prolactin stops and the rebound begins, the hypothalamus secretes a gonadotropin-releasing hormone (GnRH). The GnRH will signal the pituitary gland to produce and secrete luteinizing hormone (LH) and FSH. LH orders the production of progesterone, DHEA kickstarts the production of testosterone (this is why most men get erections in the morning) and the DHEA combined with the progesterone raises body temperature and keeps the body warm at night.

This is all a very, very healthy process, as ideally, testosterone is highest in the morning, and prolactin should not be produced in the day unless under very specific circumstances.

Monophasic sleepers will only produce a very small dose of midnight prolactin, or not get it at all, so the entire day’s hormone regulation becomes very sluggish, or doesn’t work at all! Testosterone does not get secreted until the later hours of the morning… some people will constantly produce small amounts of prolactin throughout both the night and the day to make up for lack of midnight surge which develops prolactin resistance and prolactinoma… prolactin interferes with other hormones which result in low testosterone, low estriol, low estrone, and high estradiol which results in fat gain… the dopamine system becomes exhausted and ineffective at producing GnRH resulting in low libido. Many doctors will attribute this to disease caused by general aging, but it could be a result of bad sleeping habits and forced monophasic sleep.

To reiterate, the result of no midnight prolactin surge is constantly secreted prolactin. Estradiol is needed to produce prolactin throughout the day. Estradiol is about 10 times as potent as estrone and about 80 times as potent as estriol in its estrogenic effect, so is there an estrogen-type imbalance and effective estrogen power to effective testosterone power becomes imbalanced. It is most noticeable in women as they age. Not only does high effective estrogen cause fat gain in the belly, and chest areas, but it is also related to many diseases.

TL: DR

Early to bed, early to rise makes a man healthy wealthy and wise… some hours at night and a nap in the day keep the diseases at bay. Have 3-4 hours of sleep around dusk, and some sleep around dawn.