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 = Science hasn't figured it out yet, but there is a theory called the [sleep cycle hypothesis](_URL_1_). Some scientists think that the cycle happens roughly 90 minutes after you enter your REM cycle, which is the first sleep you're likely to remember. Therefore, the cycle doesn't bother you during your sleep so much as when you wake up. EDIT: This article talks about why you feel refreshed after waking up naturally, and the benefits of the sleep cycle, as well as a lot of other things that may be made more clear to you in the future. If you're interested, check out /r/askscience. If your questions are specific to what you find interesting, then I recommend reading it. If you find something very interesting, then you can ask if you should ask again. Just a suggestion. There are probably better sources available to the community, too. I recommend doing it yourself, because I find it fascinating and useful for finding new sources when I go back to Google to check if there are any other questions I didn't understand. **EDIT 2: Just to clear things up, you'll find it takes me almost 8 hours 

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---environment and readjust body position before falling back asleep. Sleepers typically awaken soon after the end of a REM phase or sometimes in the middle of REM. Internal circadian indicators, along with successful reduction of homeostatic sleep need, typically bring about awakening and the end of the sleep cycle. Awakening involves heightened electrical activation in the brain, beginning with the thalamus and spreading throughout the cortex. During a night's sleep, a small amount of time is usually spent in a waking state. As measured by electroencephalography, young females are awake for 0–1% of the larger sleeping period; young males are awake for 0–2%. In adults, wakefulness increases, especially in later cycles. One study found 3% awake time in the first ninety-minute sleep cycle, 8% in the second, 10% in the third, 12% in the fourth, and 13–14% in the fifth. Most of this awake time occurred shortly after REM sleep. Today, many humans wake up with an alarm clock; however, people can also reliably wake themselves up at a specific time with no need for an alarm. Many sleep quite differently on workdays versus days off, a pattern which can lead to chronic circadian desynchronization. Many people regularly look at television and other screens before going to bed, a factor which may exacerbate 

RETRIEVAL #2 = Physiological psychology---and movement. On average, an adult human sleeps between seven and eight hours per night. There is a minute percentage that sleeps less than five to six hours, which is also a symptom of sleep deprivation, and an even smaller percentage of people who sleep more than ten hours a day. Oversleeping has been shown to have a correlation with higher mortality. There are no benefits to oversleeping and it can result in sleep inertia, which is the feeling of drowsiness for a period of time after waking. There are two phases of sleep: rapid eye movement (REM) and Non-REM sleep (NREM). REM sleep is the less restful stage in which you dream and experience muscle movements or twitches. Also during this stage in sleep, a person's heart rate and breathing are typically irregular. Non-REM sleep, also sometimes referred to as slow-wave sleep, is associated with deep sleep. The body's blood pressure, heart rate, and breathing are generally significantly decreased compared to an alert state. Dreaming can occur in this state; however a person is not able to remember them due to how deep in sleep they are and the inability for consolidation to occur in memory. REM cycles typically occur in 90 minute intervals and increase in length as the amount of sleep in one session progresses. In a typical night's rest, a person will have about four to 

RETRIEVAL #3 = Sleep deprivation---changes primarily occur in two regions: the thalamus, a structure involved in alertness and attention; and the prefrontal cortex, a region sub-serving alertness, attention, and higher-order cognitive processes. This was the finding of an American study in 2000. Seventeen men in their 20s were tested. Sleep deprivation was progressive with measurements of glucose (absolute regional CMRglu), cognitive performance, alertness, mood, and subjective experiences collected after 0, 24, 48, and 72 h of sleep deprivation. Additional measures of alertness, cognitive performance, and mood were collected at fixed intervals. PET scans were used and attention was paid to the circadian rhythm of cognitive performance. A noted 2002 University of California animal study indicated that non-rapid eye movement sleep (NREM) is necessary for turning off neurotransmitters and allowing their receptors to "rest" and regain sensitivity which allows monoamines (norepinephrine, serotonin and histamine) to be effective at naturally produced levels. This leads to improved regulation of mood and increased learning ability. The study also found that rapid eye movement sleep (REM) deprivation may alleviate clinical depression because it mimics selective serotonin reuptake inhibitors (SSRIs). This is because the natural decrease in monoamines during REM is not allowed to occur, which causes the concentration 

RETRIEVAL #4 = Sleep---is for a person to fall asleep and remain in a sleeping state, and how many times they wake up during a single night. Poor sleep quality disrupts the cycle of transition between the different stages of sleep. Subjective sleep quality in turn refers to a sense of being rested and regenerated after awaking from sleep. A study by A. Harvey et al. (2002) found that insomniacs were more demanding in their evaluations of sleep quality than individuals who had no sleep problems. Homeostatic sleep propensity (the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep. Along with corresponding messages from the circadian clock, this tells the body it needs to sleep. A person who regularly awakens at an early hour will generally not be able to sleep much later than his or her normal waking time, even if moderately sleep-deprived. The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening: maximum concentration of the hormone melatonin, and minimum core body temperature. Section::::Ideal duration. Human sleep needs vary by age and amongst individuals; sleep is considered to be adequate when there is no daytime sleepiness or dysfunction. Moreover, self-reported sleep duration is only moderately correlated with actual sleep time as 

RETRIEVAL #5 = Wakefulness---Wakefulness Wakefulness is a daily recurring brain state and state of consciousness in which an individual is conscious and engages in coherent cognitive and behavioral responses to the external world. Being awake is the opposite of the state of being asleep in which most external inputs to the brain are excluded from neural processing. Section::::Effects upon the brain. The longer the brain has been awake, the greater the synchronous firing rates of cerebral cortex neurons. After sustained periods of sleep, both the speed and synchronicity of the neurons firing are shown to decrease. Another effect of wakefulness is the reduction of glycogen held in the astrocytes, which supply energy to the neurons. Studies have shown that one of sleep's underlying functions is to replenish this glycogen energy source. Section::::Maintenance by the brain. Wakefulness is produced by a complex interaction between multiple neurotransmitter systems arising in the brainstem and ascending through the midbrain, hypothalamus, thalamus and basal forebrain. The posterior hypothalamus plays a key role in the maintenance of the cortical activation that underlies wakefulness. Several systems originating in this part of the brain control the shift from wakefulness into sleep and sleep into wakefulness. Histamine neurons in the tuberomammillary nucleus and nearby adjacent posterior hypothalamus project to the entire brain and are the most wake 

RETRIEVAL #6 = Sleep inertia---simple motor task; the accuracy of sensory and motor functioning is more impaired by sleep inertia as compared to sheer speed. In order to measure the cognitive and motor deficiencies associated with sleep inertia, a battery of tests may be utilized including psychomotor vigilance task, descending subtraction task, auditory reaction time task, and the finger tapping task. Section::::Causes. BULLET::::- Studies have shown that abrupt awakening during stage 3 sleep, slow-wave sleep (SWS), produces more sleep inertia than awakening during sleep stages 1, 2 or REM sleep BULLET::::- Prior sleep deprivation increases the percentage of time spent in slow-wave sleep (SWS). Therefore, an individual who was previously sleep deprived will have a greater chance of experiencing sleep inertia. BULLET::::- Adenosine levels in the brain progressively increase with sleep deprivation, and return to normal during sleep. Upon awakening with sleep deprivation, high amounts of adenosine will be bound to receptors in the brain, neural activity slows down, and a feeling of tiredness will result BULLET::::- Studies show that individuals express a lack of blood flow to the brain upon awakening. Levels of cerebral blood flow (CBF) and cerebral blood flow velocities (CBFV) will take up to 30 minutes to increase and reach daytime levels. Studies using advanced