Solving sleep: just a toe-dipping

[For the past few months I’ve been un­der­tak­ing a mostly in­de­pen­dent study of sleep, and look­ing to build a co­her­ent model of what sleep does and find ways to op­ti­mize it. I’d like to write a se­ries of posts out­lin­ing my find­ings and hy­pothe­ses. I’m not sure if this is the best venue for such a pro­ject, and I’d like to gauge com­mu­nity in­ter­est. This first post is a brief overview of one im­por­tant as­pect of sleep, with a few re­lated points of recom­men­da­tion, to provide some back­ground knowl­edge.]

In the quest to be­come more effec­tive and pro­duc­tive, sleep is an enor­mously im­por­tant pro­cess to op­ti­mize. Most of us spend (or at least think we should spend) 7.5 to 8.5 hours in bed ev­ery night, a third of a 24 hour day. Not sleep­ing well and not sleep­ing suffi­ciently have known and large draw­backs, in­clud­ing de­creased at­ten­tion, greater ir­ri­ta­bil­ity, de­pressed im­mune func­tion, and gen­er­ally weak­ened cog­ni­tive abil­ity. If you’re look­ing for more time, ei­ther for sub­jec­tive life-ex­ten­sion, or so that you can get more done in a day, tak­ing steps to sleep most effi­ciently, so as to not spend more than the re­quired amount of time in bed and to get the full benefit of the rest, is of high value.

Un­der­stand­ing the in­ner mechanisms of this pro­cess, can let us work around them. Sleep, baf­fling as it is (and it is ex­tremely baf­fling), is not a black box. Know­ing how it works, you can or­ga­nize your be­hav­ior to ac­com­mo­date the world as it is, just as tak­ing ad­van­tage of the prin­ci­ples of aero­dy­nam­ics, thrust, and lift, en­ables one to build an air­plane.

The most im­por­tant thing to know about sleep and wake­ful­ness is that it is the re­sult of a dual pro­cess: how alert a per­son feels is de­ter­mined by two differ­ent and op­po­site func­tions. The first is termed the home­o­static sleep drive (also, home­o­static drive, sleep load, sleep pres­sure, and pro­cess S), which is de­ter­mined solely by how long it has been since an in­di­vi­d­ual has last slept fully. The longer he/​she’s been awake, the greater his/​her sleep drive. It is the brain’s biolog­i­cal need to sleep. Just as suffi­cient need for calories pro­duces hunger, suffi­cient sleep-drive pro­duces sleep­iness. Sleep­ing de­creases sleep drive, and sleep drive drops faster (when sleep­ing) then it rises (when awake).

Neu­ro­science is com­pli­cated, but it seems the chem­i­cal cor­re­late of sleep drive is the build-up of adeno­sine in the basal fore­brain and this is used as the brain’s in­ter­nal mea­sure of how badly one needs sleep.1 (Caf­feine makes us feel alert by com­pet­ing with adeno­sine for bond­ing sites and thereby in­hibit­ing re­up­take.)

This is only half the story, how­ever. Adeno­sine lev­els are much higher (and sleep drive cor­re­spond­ingly lower) in the evening, when one has been awake for a while, than in the mid­dle of the night, when one has just slept for sev­eral hours. If sleep­iness were only de­ter­mined by sleep drive, you would have a much more frag­mented sleep: sleep­ing sev­eral times dur­ing the day, and wak­ing up sev­eral times dur­ing the night. In­stead, hu­mans typ­i­cally stay awake through the day, and sleep through the whole night. This is due to the sec­ond in­fluence on wake­ful­ness: the cir­ca­dian alert­ing sig­nal.

For most of hu­man his­tory, there was lit­tle that could be done at night. Dark­ness made it much more difficult to hunt or gather than it was dur­ing day. Given that the brain re­quires some frac­tion of the ny­chthe­meron (mean­ing a 24-hour pe­riod) asleep, it is evolu­tion­ar­ily prefer­able to con­cen­trate that frac­tion of of the ny­chthe­meron in the night­time, free­ing the day to do other things. For this rea­son, there is also a cycli­cal com­po­nent to one’s alert­ness: in­de­pen­dent of how long it has been since an in­di­vi­d­ual has slept, there will be times in the ny­chthe­meron when he/​she will feel more or less tired.

Roughly, the cir­ca­dian alert­ing sig­nal (also known as pro­cess C) coun­ters the sleep-drive, so that as sleep drive builds up dur­ing the day, alert­ness stays con­stant, and as sleep drive in­creases over the course of the night, the in­di­vi­d­ual will stay asleep.

The alert­ing sig­nal is syn­chro­nized to cir­ca­dian rhythms, which are in turn at­tuned to light ex­po­sure. The cir­ca­dian clock is set so that the alert­ing sig­nal be­gins to in­crease again (af­ter a night of sleep) at the time when the op­tic nerve is first ex­posed to light in the morn­ing (or rather, when the the op­tic nerve has ha­bit­u­ally been first ex­posed to light, since it takes up to a week to re­set cir­ca­dian rhythms), and in­creases with the sleep drive un­til about 14 hours later (from the point that the alert­ing sig­nal started ris­ing).

This is why if you pull an “all-nighter” you might find it difficult to fall asleep dur­ing the fol­low­ing day, even if you feel ex­hausted. Your sleep drive is high, but the alert­ing sig­nal is trig­ger­ing wake­ful­ness, which makes it hard to fall asleep.

For un­known rea­sons, there is a dip in the cir­ca­dian alert­ing about 8 hours af­ter the be­gin­ning of the cy­cle. This is why peo­ple some­times ex­pe­rience that “2:30 feel­ing.” This is also the time at which bipha­sic cul­tures typ­i­cally have an af­ter­noon siesta. This is use­ful to know, be­cause this is the best time to take a nap if you want to make up sleep missed the night be­fore.

The neu­ro­chem­istry of the cir­ca­dian alert­ing sig­nal is more com­plex than that of the sleep drive, but one of the key chem­i­cals of pro­cess C is mela­tonin, which is se­creted by the pineal gland about 12 hours af­ter the start of the cir­ca­dian cy­cle (two hours be­fore ha­bit­ual bed­time). It is mildly sleep-in­duc­ing.

This is why tak­ing mela­tonin tablets be­fore is recom­mended by gw­ern and oth­ers. I sec­ond this recom­men­da­tion. Though not FDA-ap­proved, there seem to be lit­tle in the way of nega­tive side effects and they make it much eas­ier to fall asleep.

The nat­u­ral re­lease of mela­tonin is in­hibited by light, and in par­tic­u­lar blue light (which is why it is benefi­cial ap­pli­ca­tions to red-shift the light of their com­puter screens, like flux or reds.shift, or wear red-tinted gog­gles, be­fore bed). By limit­ing light ex­po­sure in the late evening you al­low nat­u­ral mela­tonin se­cre­tion, which both stim­u­lates sleep and pre­vents the cir­ca­dian clock from shift­ing (which would make it even more difficult to fall asleep the fol­low­ing night). Re­cent stud­ies have shown bright screens ant night do demon­stra­bly dis­rupt sleep.2

The thing that in­ter­ests me about this fact that alert­ness is con­trol­led by both pro­cess S and pro­cess C, is that it may be pos­si­ble to mod­u­late each of those pro­cesses in­de­pen­dently. It would be enor­mously use­ful to be able to “turn off” the cir­ca­dian alert­ing sig­nal on de­mand, so that a per­son can fall asleep at any time off the day, to make up sleep loss when­ever is con­ve­nient. In­stead of ac­com­mo­dat­ing cir­ca­dian rhythms when schedul­ing, we could ad­just the cir­ca­dian effect to bet­ter fit our lives. When you know you’ll need to be awake all night, for in­stance, you could turn off the alert­ing sig­nal around mid­day and sleep un­til your sleep drive is re­set. In fact, is sus­pect that those peo­ple who are able to live suc­cess­fully on a polypha­sic sleep sched­ule get the benefits by re­train­ing the cir­ca­dian in­fluence. In the com­ing posts, I want to out­line a few of the pos­si­bil­ities and (sig­nifi­cant) prob­lems in that di­rec­tion.

1 Blanco-Cen­tu­rion, C., Xu, M., Murillo-Ro­driguez, E., Gerashchenko, D., Shi­ro­mani, A., Salin-Pas­cual, Shi­ro­mani, P. (2006). Adeno­sine and Sleep Homeosta­sis in the Basal Fore­brain. Jour­nal of Neu­ro­science, 8092-8100.

2Na­tional Sleep Foun­da­tion. (2011, March 7). An­nual Sleep in Amer­ica Poll Ex­plor­ing Con­nec­tions with Com­mu­ni­ca­tions Tech­nol­ogy Use and Sleep. Retrieved Au­gust 17, 2011, from http://​​www.sleep­foun­da­​​ar­ti­cle/​​press-re­lease/​​an­nual-sleep-amer­ica-poll- ex­plor­ing-con­nec­tions-com­mu­ni­ca­tions-tech­nol­ogy-use-.