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← A walk through the stages of sleep

Did you know you go on a journey every night after you close your eyes? Sleep scientist Matt Walker breaks down the difference between REM (Rapid-Eye Movement) and non-REM sleep, what occurs during each stage of sleep -- and why it's important to get enough of both.

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Showing Revision 5 created 09/02/2020 by Erin Gregory.

  1. Sleep is perhaps the
    single most effective thing
  2. that we can do each and every day
  3. to reset the health
    of our brain and our body.
  4. And by understanding a little bit more
    about what sleep is,
  5. perhaps we can get the chance to improve
    both the quantity and the quality
  6. of our sleep.
  7. [Sleeping with Science]

  8. (Music)

  9. So, exactly what is sleep?

  10. Well, sleep, at least in human beings,
  11. is subdivided into two main types.
  12. On the one hand, we have
    non-rapid eye movement sleep,
  13. or non-REM sleep for short.
  14. But on the other hand,
  15. we have rapid eye movement
    sleep, or REM sleep.
  16. And non-REM sleep has been
    further subdivided

  17. into four separate stages,
  18. unimaginatively called
    stages one through four,
  19. increasing in their depth of sleep.
  20. And as we go into those light stages
    of non-REM sleep,
  21. your heart rate starts to decrease,
  22. your body temperature starts to drop
  23. and your electrical brain wave activity
    starts to slow down.
  24. But as we move into deeper
    non-rapid eye movement sleep,

  25. stages three and four,
  26. now all of a sudden the brain erupts
  27. with these huge, big,
    powerful brain waves.
  28. The body is actually recharged
    in terms of its immune system.
  29. We also get this beautiful overhaul
    of our cardiovascular system.
  30. And, in fact, upstairs in the brain,
  31. deep non-REM sleep
    will help consolidate memories
  32. and fixate them into the neural
    architecture of the brain.
  33. So that's non-REM sleep.

  34. But let's come on to REM sleep,
  35. which is the other main type of sleep.
  36. And it's during REM sleep
    when we principally have the most vivid,
  37. the most hallucinogenic types of dreams.
  38. The brain wave activity
    actually starts to speed up again.
  39. It's during REM sleep that we receive
    almost a form of emotional first aid.
  40. And it's also during REM sleep
    where we get a boost for creativity,
  41. that it stitches information together
  42. so that we wake up with solutions
  43. to previously difficult problems
    that we were facing.
  44. Coming back to these two types of sleep,

  45. it turns out that non-REM
    and REM will play out
  46. in a battle for brain domination
    throughout the night,
  47. and that cerebral war
    is going to be won and lost
  48. every 90 minutes,
  49. and then it's going to be
    replayed every 90 minutes.
  50. And what this produces is a standard
    cycling architecture of human sleep,
  51. a standard 90-minute cycle.
  52. But what's different, however,

  53. is that the ratio of non-REM to REM
    within those 90-minute cycles
  54. changes as we move across the night,
  55. such that in the first half the night,
  56. the majority of those 90-minute cycles
  57. are comprised of lots
    of deep non-REM sleep,
  58. particularly stages three and four
    of non-REM sleep.
  59. But as we push through
    to the second half of the night,
  60. now that seesaw balance
    actually shifts over,
  61. and instead, most of those
    90-minute cycles
  62. are comprised of a lot more
    rapid eye movement sleep, or dream sleep,
  63. as well as stage-two non-REM sleep,
  64. that lighter form of non-REM sleep.
  65. And it turns out
    that there are implications

  66. for understanding how sleep
    is structured in this way.
  67. Let's take someone who typically
    goes to bed at 10pm,
  68. and they wake up at 6am,
  69. so they have an eight-hour sleep window.
  70. But this morning,
    they have to wake up early
  71. for an early morning meeting,
  72. or they want to get
    a jump start on the day
  73. to get to the gym.
  74. And as a consequence, they have to wake up
    at 4am in the morning,
  75. rather than 6am in the morning.
  76. How much sleep have they actually lost?
  77. Two hours out of
    an eight-hour night of sleep
  78. means that they've lost
    25 percent of their sleep.
  79. Well, yes and no.
  80. They have lost 25 percent
    of all of their sleep,
  81. but because REM sleep comes
    mostly in the second half of the night
  82. and particularly in those last few hours,
  83. they may have lost perhaps
    50, 60, maybe even 70 percent
  84. of all of their REM sleep.
  85. So there are real consequences
    to understanding what sleep is

  86. and how sleep is structured.
  87. And we'll learn all about the benefits
    of these different stages of sleep
  88. and the detriments that happen
    when we don't get enough of them
  89. in subsequent episodes.