0:00:00.130,0:00:02.936 Sleep is perhaps the[br]single most effective thing 0:00:02.960,0:00:05.073 that we can do each and every day 0:00:05.097,0:00:08.816 to reset the health[br]of our brain and our body. 0:00:08.840,0:00:12.756 And by understanding a little bit more[br]about what sleep is, 0:00:12.780,0:00:17.238 perhaps we can get the chance to improve[br]both the quantity and the quality 0:00:17.262,0:00:18.269 of our sleep. 0:00:18.299,0:00:20.136 [Sleeping with Science] 0:00:20.136,0:00:21.800 (Music) 0:00:23.130,0:00:25.807 So, exactly what is sleep? 0:00:26.270,0:00:28.914 Well, sleep, at least in human beings, 0:00:28.938,0:00:31.926 is subdivided into two main types. 0:00:31.950,0:00:35.306 On the one hand, we have[br]non-rapid eye movement sleep, 0:00:35.330,0:00:37.026 or non-REM sleep for short. 0:00:37.050,0:00:38.216 But on the other hand, 0:00:38.240,0:00:42.226 we have rapid eye movement[br]sleep, or REM sleep. 0:00:42.250,0:00:45.658 And non-REM sleep has been[br]further subdivided 0:00:45.682,0:00:47.841 into four separate stages, 0:00:47.865,0:00:51.866 unimaginatively called[br]stages one through four, 0:00:51.890,0:00:54.406 increasing in their depth of sleep. 0:00:54.430,0:00:58.226 And as we go into those light stages[br]of non-REM sleep, 0:00:58.250,0:01:01.026 your heart rate starts to decrease, 0:01:01.050,0:01:03.186 your body temperature starts to drop 0:01:03.210,0:01:07.236 and your electrical brain wave activity[br]starts to slow down. 0:01:07.260,0:01:11.416 But as we move into deeper[br]non-rapid eye movement sleep, 0:01:11.440,0:01:13.316 stages three and four, 0:01:13.340,0:01:16.326 now all of a sudden the brain erupts 0:01:16.350,0:01:20.065 with these huge, big,[br]powerful brain waves. 0:01:20.470,0:01:25.336 The body is actually recharged[br]in terms of its immune system. 0:01:25.360,0:01:29.912 We also get this beautiful overhaul[br]of our cardiovascular system. 0:01:30.300,0:01:32.706 And, in fact, upstairs in the brain, 0:01:32.730,0:01:35.976 deep non-REM sleep[br]will help consolidate memories 0:01:36.000,0:01:39.676 and fixate them into the neural[br]architecture of the brain. 0:01:39.700,0:01:42.041 So that's non-REM sleep. 0:01:42.065,0:01:44.526 But let's come on to REM sleep, 0:01:44.550,0:01:46.736 which is the other main type of sleep. 0:01:46.760,0:01:51.246 And it's during REM sleep[br]when we principally have the most vivid, 0:01:51.270,0:01:54.136 the most hallucinogenic types of dreams. 0:01:54.160,0:01:58.236 The brain wave activity[br]actually starts to speed up again. 0:01:58.260,0:02:04.876 It's during REM sleep that we receive[br]almost a form of emotional first aid. 0:02:04.900,0:02:09.376 And it's also during REM sleep[br]where we get a boost for creativity, 0:02:09.400,0:02:12.176 that it stitches information together 0:02:12.200,0:02:14.709 so that we wake up with solutions 0:02:14.733,0:02:18.176 to previously difficult problems[br]that we were facing. 0:02:18.200,0:02:20.556 Coming back to these two types of sleep, 0:02:20.580,0:02:24.836 it turns out that non-REM[br]and REM will play out 0:02:24.860,0:02:28.616 in a battle for brain domination[br]throughout the night, 0:02:28.640,0:02:33.046 and that cerebral war[br]is going to be won and lost 0:02:33.070,0:02:34.836 every 90 minutes, 0:02:34.860,0:02:38.059 and then it's going to be[br]replayed every 90 minutes. 0:02:38.544,0:02:44.252 And what this produces is a standard[br]cycling architecture of human sleep, 0:02:44.276,0:02:46.180 a standard 90-minute cycle. 0:02:46.759,0:02:48.486 But what's different, however, 0:02:48.510,0:02:54.356 is that the ratio of non-REM to REM[br]within those 90-minute cycles 0:02:54.380,0:02:57.356 changes as we move across the night, 0:02:57.380,0:02:59.826 such that in the first half the night, 0:02:59.850,0:03:02.606 the majority of those 90-minute cycles 0:03:02.630,0:03:06.006 are comprised of lots[br]of deep non-REM sleep, 0:03:06.030,0:03:09.966 particularly stages three and four[br]of non-REM sleep. 0:03:09.990,0:03:13.346 But as we push through[br]to the second half of the night, 0:03:13.370,0:03:17.476 now that seesaw balance[br]actually shifts over, 0:03:17.500,0:03:20.796 and instead, most of those[br]90-minute cycles 0:03:20.820,0:03:25.530 are comprised of a lot more[br]rapid eye movement sleep, or dream sleep, 0:03:25.554,0:03:28.506 as well as stage-two non-REM sleep, 0:03:28.530,0:03:30.696 that lighter form of non-REM sleep. 0:03:30.720,0:03:34.268 And it turns out[br]that there are implications 0:03:34.292,0:03:38.025 for understanding how sleep[br]is structured in this way. 0:03:38.049,0:03:42.786 Let's take someone who typically[br]goes to bed at 10pm, 0:03:42.810,0:03:44.817 and they wake up at 6am, 0:03:44.841,0:03:47.068 so they have an eight-hour sleep window. 0:03:47.488,0:03:49.998 But this morning,[br]they have to wake up early 0:03:50.022,0:03:52.336 for an early morning meeting, 0:03:52.360,0:03:54.426 or they want to get[br]a jump start on the day 0:03:54.450,0:03:55.816 to get to the gym. 0:03:55.840,0:03:59.796 And as a consequence, they have to wake up[br]at 4am in the morning, 0:03:59.820,0:04:02.036 rather than 6am in the morning. 0:04:02.060,0:04:04.376 How much sleep have they actually lost? 0:04:04.400,0:04:07.026 Two hours out of[br]an eight-hour night of sleep 0:04:07.050,0:04:11.107 means that they've lost[br]25 percent of their sleep. 0:04:11.559,0:04:13.771 Well, yes and no. 0:04:13.795,0:04:17.116 They have lost 25 percent[br]of all of their sleep, 0:04:17.140,0:04:21.530 but because REM sleep comes[br]mostly in the second half of the night 0:04:21.554,0:04:23.546 and particularly in those last few hours, 0:04:23.570,0:04:28.931 they may have lost perhaps[br]50, 60, maybe even 70 percent 0:04:28.955,0:04:30.746 of all of their REM sleep. 0:04:30.770,0:04:36.040 So there are real consequences[br]to understanding what sleep is 0:04:36.064,0:04:37.786 and how sleep is structured. 0:04:37.810,0:04:42.066 And we'll learn all about the benefits[br]of these different stages of sleep 0:04:42.090,0:04:45.613 and the detriments that happen[br]when we don't get enough of them 0:04:45.637,0:04:47.277 in subsequent episodes.