QUESTION = What's the science behind waking up in the middle of the night after only a few hours of sleep feeling rested, only to get a full eight and wake up tired? GENERATION = Sleep cycles. You go through multiple sleep cycles every night. REM (rapid eye movement) sleep, and deep sleep. If you wake up in the middle of one of these cycles, you feel like shit. If you wake up during a REM cycle, you feel refreshed. If you wake up during a deep sleep cycle, you feel like shit. If you wake up during a light sleep cycle, you feel refreshed. If you wake up during a deep sleep cycle, you feel like shit. It's not a perfect science, but you get the idea. You can also learn to get better at sleeping in between cycles, by learning to sleep in intervals of 90 minutes. If you don't want to sleep for 8 hours, try 8, 3, 4, 3, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 RETRIEVAL #0 = Sleep cycle---to the end of the next, or from the beginning of REM, or from the beginning of non-REM stage 2. (The decision of how to mark the periods makes a difference for research purposes because of the unavoidable inclusion or exclusion of the night’s first NREM or its final REM phase if directly preceding awakening.) A 7–8-hour sleep probably includes five cycles, the middle two of which tend to be longer. REM takes up more of the cycle as the night goes on. Section::::Awakening. Unprovoked awakening occurs most commonly during or after a period of REM sleep, as body temperature is rising. Section::::Continuation during wakefulness. Ernest Hartmann found in 1968 that humans seem to continue a roughly 90-minute ultradian rhythm throughout a 24-hour day, whether they are asleep or awake. According to this hypothesis, during the period of this cycle corresponding with REM, people tend to daydream more and show less muscle tone. Kleitman and others following have referred to this rhythm as the basic rest–activity cycle, of which the "sleep cycle" would be a manifestation. A difficulty for this theory is the fact that a long non-REM phase almost always precedes REM, regardless of when in the cycle a person falls asleep. Section::::Alteration. The sleep cycle has proven resistant to RETRIEVAL #1 = Sleep cycle---start to decrease at this stage. N2 is when the person experiences a light sleep. Eye movement has stopped by this time. Brain wave frequency and muscle tonus is decreased. The heart rate and body temperature goes down. N3 or even N4 are the most difficult stages to be awakened. Every part of the body is now relaxed, breathing, blood pressure and body temperature are reduced. The National Sleep Foundation discusses the different stages of NREM sleep and their importance. They describe REM sleep as "A unique state, in which dreams usually occur. The brain is awake and body paralyzed." This unique stage is usually when the person is in the deepest stage of sleep and dreams. The figure of 90 minutes for the average length of a sleep cycle was popularized by Nathaniel Kleitman around 1963. Other sources give 90–110 minutes or 80–120 minutes. In infants the sleep cycle lasts about 50–60 minutes; average length increases as the human grows into adulthood. In cats the sleep cycle lasts about 30 minutes, in rats about 12 minutes, and in elephants up to 120 minutes. (In this regard the ontogeny of the sleep cycle appears proportionate with metabolic processes, which vary in proportion with organism size. However, shorter sleep cycles detected in some elephants complicate this theory.) The cycle can be defined as lasting from the end of one REM period RETRIEVAL #2 = Sleep Like a Tiger---Section::::Plot. Starting with the old adage "Once there was a little girl..." you come across a little girl and her parents, all wearing crowns. The little girl is wide awake claiming she is not tired. Her parents take this almost as a regular occurrence and require that she at least put on her pajamas, wash her face, brush her teeth. The little girl keeps repeating that she is not tired but climbs into bed and under the sheets since that is where she is the most comfortable. She then proceeds to ask her parents if everything thing goes to sleep. The parents then proceed to go through a list of animals and their habits of sleeping. They discuss the family dog, the cat, bats, snails, bears, and whales. The little girl proclaims that she knows an animal that sleeps a lot. We then finally come across the tiger in the jungle, where the little girl tells her parents that he sleeps to stay strong. The parents agree that sleep is good to stay strong then say goodnight even though the little girl claims she is still not sleepy. Her parents know what will happen and agree that she can stay awake all night long. After the long discussion, the little girl goes through all the processes that the animals go through and ends up fast asleep. Section::::Critical reception. "Sleep Like RETRIEVAL #3 = Sleep inertia---Flying Safely Article on Napping RETRIEVAL #4 = Wake Up---Wake Up To wake up is to emerge from sleep. Wake up may also refer to: Section::::Television. BULLET::::- "Wake Up" (TV program), a former Australian breakfast TV show BULLET::::- "Wake-Up", an episode of the television series "Teletubbies" BULLET::::- "Wake Up", a 2014 episode of the animated television series "Adventure Time" BULLET::::- ""Wake Up"" (TV series), a medical drama series from Taiwan BULLET::::- "Wake Up" ("Agents of S.H.I.E.L.D."), a 2017 episode from the fourth season of "Agents of S.H.I.E.L.D." BULLET::::- "Wake Up" ("Supergirl"), a 2017 episode from the third season of "Supergirl" Section::::Music. Section::::Music.:Albums. BULLET::::- "Wake Up!" (The Boo Radleys album), 1995 BULLET::::- "Wake Up" (BTS album), 2014 BULLET::::- "Wake Up" (Jessica Andersson album), 2009 BULLET::::- "Wake Up!" (John Legend and The Roots album), 2010 BULLET::: RETRIEVAL #5 = Second wind (sleep)---that their performance, though an improvement considering their state of sleep deprivation, were below what it would be had they slept.) Another study found significant improvement in the performance of 31 adults on various neurobehavioral tests after the onset of the so-called "wake maintenance zone" as compared to their performance just three hours prior, despite the fact that the subjects have been awake longer. The improvement as test subjects caught another wind was even more pronounced on the second day of extended wakefulness. Section::::Characteristics.:Duration. The wake maintenance zone generally lasts 2 to 3 hours, during which one is less inclined to fall asleep. While potentially useful for completing urgent tasks, it may have a potentially unwanted side-effect of keeping one awake for several hours after the task has been completed. The hypervigilance and stimulation brought on by a second wind can cause fatigue, which, in the case of infants, can be literally painful. Thus, an infant may begin crying when sleep habits are disrupted. One can avoid "getting a second wind" by practicing proper sleep hygiene. Section::::Characteristics.:"Fifth day turning point". Multiple studies have observed that individuals subjected to total sleep deprivation for extended periods spanning multiple days may feel "helplessly sleepy" up until the fifth day, upon which all observed individuals would feel what may be described as a second wind. This particular form of the experience has RETRIEVAL #6 = Sleep and weight---in subjects' BMIs. Metabolism Metabolism involves two biochemical processes that occur in living organisms. The first is anabolism, which refers to the buildup of molecules. The second is catabolism, the breakdown of molecules. These two processes work to regulate the amount of energy the body uses to maintain itself. During non-REM sleep, metabolic rate and brain temperature are lowered to deal with damages that may have occurred during time of wakefulness. Sleep is important in regulating metabolism. Mammalian sleep can be sub-divided into two distinct phases - REM (rapid eye movement) and non-REM (NREM) sleep. In humans, NREM sleep has four stages, where the third and fourth stages are considered slow-wave sleep (SWS). SWS is considered deep sleep, when metabolism is least active. In normal metabolic function, the pancreas releases insulin after blood glucose levels raise. Insulin signals muscle and fat cells to absorb glucose from food. As a result, blood glucose levels return to normal. Sleep loss can affect the basic metabolic functions of storing carbohydrates and regulating hormones. Reduction of sleep from eight hours to four hours produces changes in glucose tolerance and endocrine function. Researchers from the University of Chicago Medical Center followed 11 healthy young men for 16 consecutive nights