Sleep deprivation isn't always obvious.

"Sleep debt" (or sleep deficit) is the difference between the amount of sleep you should be getting and the amount you actually get. This difference represents a deficit which increases every time we skim some extra minutes off our nightly sleep.
"People accumulate sleep debt surreptitiously," says psychiatrist William C. Dement, founder of the Stanford University Sleep Clinic. In fact, studies have shown that such short-term sleep deprivation leads to a "foggy brain", worsened vision, impaired driving and problems in short-term memory. Long-term effects include obesity, insulin resistance, and heart disease.
And most Americans suffer from chronic deprivation. A 2005 survey by the National Sleep Foundation reports that, on average, Americans sleep 6.9 hours per night -6.8 hours during the week and 7.4 hours on the weekends: experts recommend eight hours of sleep per night.
The problem is that after a long period of sleep deprivation, you stop realizing how tired you actually are.

In a study by the University of Pennsylvania School of Medicine, researchers followed three groups of subjects for 14 days; one group slept for eight hours a night, the second group slept for six hours a night, and the third group slept just four hours a night. Cognitive tests after the two-week period showed that the people with only six hours of sleep a night showed similar reaction times as people with a blood alcohol content of 0.1 percent; a level which is considered legally impaired.

Other studies have indicated that a consistent lack of sleep may cause permanent damage to your brain. 
One in particular, also conducted by the University of Pennsylvania School of Medicine and published in The Journal of Neuroscience looked at lab mice that were kept awake to replicate the kind of sleep loss common in modern life, through night shifts or long hours in the office.
The team studied certain brain cells which are involved in keeping the brain alert. After several days of sleep patterns similar to those followed by night workers - three days of night shifts with only four to five hours sleep in 24 hours - the mice lost 25% of the brain cells in vital parts of the brain stem.

After only a night or two of sleep deficit, a few nights of adequate sleep are usually enough to reverse the effects and "recharge" your system. But chronic sleep deprivation may be harder to recover from.
For information on how to increase your sleep drive and improve your waking hours, take a look at our  sleep hygiene recommendations. If you have a problem with daytime sleepiness and you have increased your sleep length without improving your daytime sleepiness, then you should be evaluated by a Sleep Disorders Specialist. With proper care most people can obtain the treatment they need to improve this problem.

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References:
Sleep-Deprived People Can’t Tell They’re Sleep-Deprived; Melissa Dahl, NYMag.com
Lost sleep leads to loss of brain cells, study suggests; Helen Briggs, BBC News

Fragmented sleep could be as physically harmful as a total lack of sleep.

It will come as no surprise to new parents struggling after a night of feeds or doctors on call, but being woken up in the night is as detrimental as getting just four hours of sleep. Just one night of interrupted sleep may negatively affect mood, attention span and cognitive ability according to a new study from Tel Aviv University, published in the journal Sleep Medicine.

In the last 50 years, sleep research has focused on sleep deprivation, and practically ignored the impact of night-wakings, which is a pervasive phenomenon for people from many walks of life. Directors of the study hope that their work will bring this to the attention of scientists and clinicians, who should recognize the price paid by individuals who have to endure frequent night-wakings.

Researchers studied the sleep patterns of 61 healthy adults, 40 of which were females between the ages of 20 and 29 years old, were traced at their homes using actigraphy and sleep diaries. Subjects slept a full eight-hours one night, followed by a night of interrupted sleep in which they received four phone calls directing them to complete a brief computer exercise before returning to bed. The interruptions were designed to keep participants awake for a period of 10 to 15 minutes. The morning after both nights, the volunteers completed tasks to measure their attention span and emotional state: results proved that just one night of interrupted sleep had negative effects on mood, attention span and cognitive ability.

While they found few significant differences between interrupted sleep and sleep deprivation, the differences between the aforementioned conditions and a normal night's sleep were vast. Results indicate an increase in depression, fatigue and confusion in addition to diminished vigor and motivation when sleep is interrupted or restricted.

This means that even when people get a total of seven hours sleep a night, being forced to wake up for regular 15 minute feeds, or consultations, will leave them feeling like they had just four hours rest. “The sleep of many parents is often disrupted by external sources such as a crying baby demanding care during the night,” said Professor Avi Sadeh and a team of researchers from Tel Aviv University's School of Psychological Sciences. Doctors on call, who may receive several phone calls a night, also experience disruptions. These night wakings could be relatively short - only five to ten minutes - but they disrupt the natural sleep rhythm.

These effects accumulate and therefore the functional price new parents-who awaken three to ten times a night for months on end-pay for common infant sleep disturbance is enormous.
Besides the physical effects of interrupted sleep, parents often develop feelings of anger toward their infants and then feel guilty about these negative feelings. The findings bear relevance to substantial portions of the population whose sleep is regularly fragmented including medical students, shift workers, military personnel and parents.
Professionals as well as the general public should be aware of the detrimental effects of the various kinds of disruption in sleep on daily functioning and mood and consider countermeasures to minimise their consequences.

For more information on how you can get a good night's sleep visit our website at HoustonSleep.net.

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References:
Study: Interrupted Sleep May Be as Harmful as No Sleep at All- Melissa Hellmann, TIME Magazine July 9, 2014