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Asleep somewhere new, one brain hemisphere keeps watch -- ScienceDaily

They consistently found that on the first night in the lab, a particular network in the left hemisphere remained more active than in the right hemisphere, specifically during a deep sleep phase known as "slow-wave" sleep. When the researchers stimulated the left hemisphere with irregular beeping sounds (played in the right ear), that prompted a significantly greater likelihood of waking, and faster action upon waking, than if sounds were played in to the left ear to stimulate the right hemisphere. In other sleep phases and three other networks tested on the first night, there was no difference in alertness or activity in either hemisphere. On the second night of sleep there was no significant difference between left and right hemispheres even in the "default-mode network" of the left hemisphere, which does make a difference on the first night. The testing, in other words, pinpointed a first-night-only effect specifically in the default-mode network of the left hemisphere during the slow-wave phase.

Sleep clock is set when you wake up

Working with young fruit flies, whose neuronal system is simpler than adults with fewer cells and easier to study, the researchers found that two types of neurons, which they called dawn cells and dusk cells, maintain a continuous cycle. As the sun rises, special “timeless” proteins, as they’re called, help the dawn cells to first signal to each other and then signal to the dusk cells. Then as the sun sets, proteins help the dusk cells signal to each other and then signal back to the dawn cells. Each signal tells the cells to synchronize with each other. Together, these two distinct signals drive the daily sleep and wake cycle.
And by disrupting specific phases of sleep, the research group showed deep or slow-wave sleep was necessary for memory formation. During this stage, the brain was "replaying" the activity from earlier in the day. Prof Wen-Biao Gan, from New York University, told the BBC: "Finding out sleep promotes new connections between neurons is new, nobody knew this before. "We thought sleep helped, but it could have been other causes, and we show it really helps to make connections and that in sleep the brain is not quiet, it is replaying what happened during the day and it seems quite important for making the connections."
In one study, consuming two hundred milligrams of caffeine significantly increased the amount of time it took for people to fall asleep later that night. (An eight-ounce cup of brewed coffee contains ninety-five to two hundred milligrams of caffeine, according to the Mayo Clinic.) It also had a profound effect on the quality of that sleep: it lowered sleep efficiency; the duration of stage-two sleep (the point at which our bodies prepare to enter deep sleep); and the spectral power of delta-wave frequencies (which are closely associated with the depth and quality of sleep). Other studies have linked caffeine to diminished sleep quality and efficiency, along with an increase in the number of times people wake during the night and how tired they feel in the morning.
The study involved adults ranging in age from 20-59 years who responded to the 2007-2008 National Health and Nutrition Examination Survey (NHANES). A history of drug use was reported by 1,811 participants. Cannabis use was assessed as any history of use, age at first use and number of times used in the past month. Sleep-related problems were considered severe if they occurred at least 15 days per month. Results show that any history of cannabis use was associated with an increased likelihood of reporting difficulty falling asleep, struggling to maintain sleep, experiencing non-restorative sleep, and feeling daytime sleepiness. The strongest association was found in adults who started marijuana use before age 15; they were about twice as likely to have severe problems falling asleep, experiencing non-restorative sleep and feeling overly sleepy during the day.
Feinsilver said that while we sleep, the blood stream is cleared of a substance that researchers call “substance S.” Many believe substance S is adenosine, a byproduct of energy production that cells release into the blood throughout waking hours. He said that without sleep, the blood gets clogged with substance S, slowing a person down from head to toe. “It looks like there’s a toxic substance building up in you, where the more you’re awake, the more you see this stuff floating around the bloodstream, and the only way to get rid of it is to sleep,” he said.
In a 2012 study in the journal Neurobiology of Learning and Memory, researchers split 36 college-aged students into three groups. Each group learned a memory task, pairing words on a screen with a sound. Afterward, one group had 60 minutes to nap, another 10 minutes. The final group didn't sleep. Upon retesting, the napping groups fared better, as expected, said Sara Alger, lead author of the study and a postdoctoral research associate at the University of Notre Dame. More interesting, she noted, was that on further testing, including a week later, the 60-minute group performed far better than the 10-minute group, which now performed as poorly as the non-napping group. The researchers concluded that slow-wave sleep—only experienced by the 60-minute nappers—is necessary for memory consolidation.
For a quick boost of alertness, experts say a 10-to-20-minute power nap is adequate for getting back to work in a pinch. For cognitive memory processing, however, a 60-minute nap may do more good, Dr. Mednick said. Including slow-wave sleep helps with remembering facts, places and faces. The downside: some grogginess upon waking. Finally, the 90-minute nap will likely involve a full cycle of sleep, which aids creativity and emotional and procedural memory, such as learning how to ride a bike. Waking up after REM sleep usually means a minimal amount of sleep inertia, Dr. Mednick said. Experts say the ideal time to nap is generally between the hours of 1 p.m. and 4 p.m. Napping later in the day could interfere with nighttime sleep.
Cacioppo measured brain activity during the sleep of lonely and nonlonely people. Those who were lonely were far more prone to micro awakenings, which suggest the brain is on alert for threats throughout the night, perhaps just as earlier humans would have needed to be when separated from their tribe.