As excerpted from Dr Weissbluth's blog

Total Sleep Duration may be Misleading

March 9, 2011 by weissbluthmethod Some parents focus on total sleep duration such as saying, “He sleeps 12 hours total so I thought everything was fine.” The problem with this is that if the bedtime is too late and the naps are long (for example: 9pm to 7am and a 2 hour nap), than they may not see that a long latency to sleep or bedtime battles or night awakenings are caused by a bedtime that is too late. Also, if the nap is late (for example, 2-4pm) the parents coming home from work and getting ready for dinner, evening activities, and getting ready for bed might mask a second wind and the ill effects of running low of sleep (for example, “Between 8-9pm he’s fine as long as we are playing with him or he is watching his DVD).
Marc
-------------------------------------------------------------------------------------
This is an interesting post from Dr Weissbluth and I agree with the theory behind it.  I see on boards about my child only sleeps "X" hours in a day and is just on the "low sleep end".   BUT that is not necessarily an accurate picture.  For example, a 2 yr old that only sleeps 10 hours at night BUT used to sleep longer, that could indicate problems.  That 10 hours could be a classic sign of overtired (for example if you had closer to a 11-12 hr sleeper).  It could be things such as teething pain (especially molars as these can cause discomfort in the morning hours when in lighter sleep mode and cause to wake), sickness,etc.  In some cases, like older toddlers, it could mean daytime nap needs to be shortened a bit (BUT this is usually only when the naps are really long.  For example, those kids that were doing 3 hr naps.  I would hesitate to shorten naps too much when they are young, especially less than 2 hrs as that can easily backfire and cause more issues.  Remember Dr Weissbluth treats day and night sleep differently).  It does not necessarily mean "My child is getting older and just needed less sleep" as so many parent erroneously believe. So then the parent just puts the child to bed later thinking they only need 10 hrs of sleep and they end up with less and less sleep over time. 

And like the article points out, if the bedtime is too late the child is not truly rested and may take long daytime naps to compensate......BUT it'd be better if the night sleep was timed earlier and much longer and then the child would be more rested and not overcompensate on nap.  Also the late nap can cause issues as pointed out, as that might interfere with bedtime.  I have found from personal experience that earlier naps help preserve the earlier bedtime and keep the sleep times in sync with the body's circadian rhythm.  Dr Weissbluth has mentioned many times how sleeping outside of the circadian rhythm is not good quality sleep.  So a child sleeping from 10 PM-10 AM even is not the same quality as 7 PM-7 AM.  So that is why just looking at the total can be inaccurate.

Another example of  this being misleading is a child who skips nap.  A young preschooler, and definitely toddlers, NEED daytime rest and sleep for optimal development.  So a preschooler who sleeps a 2 hr nap and 11 hrs at night is more rested than one who skips nap and sleeps the same overall at night (even 13 hrs).  I truly believe putting them down ultraearly on no-nap days is crucial so they don't get in an overtired cycle and allows them to get caught up on the missed sleep (and that may be ok to do once in a while) but to sustain this every day would build up a sleep defiiciency.  It would not be good on a daily basis or for their optimal development.  As they get older, maybe around 4ish, that may change and you may do consider some things like having resttime some days and naps other days,etc.  But also many parents do not put their children to bed early enough when naps are skipped and this just builds up a sleep debt (and from expereince, as counterintuitive as it sounds, the later they go to bed the less sleep they'll sleep.  Versus going to bed extraearly, they'll sleep a longer night). 

 Bottom line: I find that the most useful sleep tool for mine is to look at how much sleep they are getting within 24 hrs and to make sure it is adequate and consistent.  Anytime there is a big sudden drop, it is a red flag.  Then I troubleshoot and make sure there isn't anything else going on (like illness, teething, or overtired creeping in).  But this article points out that it is the overall picture to still consider, so that is both day and night sleep and other factors.