Image Credit: Gioia Albano (

There’s an article getting a lot of attention in the media these days that discusses a possible evolutionary reason for babies waking at night.  Namely, Dr. David Haig from Harvard proposes it’s to increase the duration of post-partum amenorrhea in women resulting in greater care of the infant at hand, ensuring greater chance for survival and passing on genes

[1].  As a hypothesis, it’s quite fascinating.

Does it hold any water?  Well, for starters we know that night nursing does increase the period of post-partum amenorrhea[2].  There is also evidence that the “optimal” spacing between children is at least two years which is often attained by breastfeeding[3].  Furthermore, in many hunter-gatherer cultures where resources are not as plentiful as they are in our modern world, women who fall pregnant and give birth before their child is around three years of age often have to kill the infant as the burden of caring for two children under three is too great[4].  Thus a mother whose infant does any behaviour that delays menstruation may be helping both him/herself and the mother.  May.

The problem with the articles that are giving it press is that they are treating it as face when it really is just a hypothesis.  There are many possible reasons infants go through a period of waking more at the six month mark and Dr. Katie Hinde, also of Harvard (and of the wonderful Mammals Suck), has done a better job than I ever could highlighting these very issues in her response piece to Dr. Haig[5] so I recommend you read her response in full (it’s linked at the end and this is an open-access journal so all articles are available for free).  Briefly, some of the mentions include the need for more nursing to deal with added energy expenditures and discomfort due to the change in gut flora from the introduction of solid foods.  Add to this known cognitive leaps that occur at the same time, the fact that it is the most common time for teething to begin, and even the start of separation anxiety for some infants and you have a laundry list of other reasons as to why babies may wake and cry more in the middle of the night.

However, there’s also another very real problem with how Dr. Haig has presented his work to the public, namely that he has decided to give some additional advice based on his hypothesis.  You see, in the popular press articles, he has decided to impart on parents that given this hypothesis (which is now being spoken of as fact), sleep training is totally acceptable.  As stated in the NPR article, “But Haig hopes the idea might help relieve anxiety some parents feel when it comes to shaping the sleeping and nursing habits of their babies — including, perhaps, training babies to sleep through the night.”

This is where I really have a really big problem.  You see, it’s perfectly laudable to come up with a new hypothesis as to why infants wake at night.  To take this theory, though – this untested hypothesis – and then offer parenting advice based on it whilst ignoring the many other biologically-based theories is downright dishonest.  I want to call the media out too, but we all know that they love a good story telling parents it’s okay to leave their children to cry-it-out so I’m not sure what can be said for them except it’s business as usual.

As such, there are a couple things people need to know that I want to highlight here.  First, people need to know that there is no hard evidence to support this hypothesis.  We cannot test it explicitly (though I have a few ideas how you could assess at least the viability to a degree using long-term observational studies) so it cannot be proven or disproven.  Notably, it ignores certain critical historical elements (like the fact that people didn’t used to sleep eight hours straight and thus probably weren’t as disrupted sleep-wise as we are today) that might make it moot.  If historically sleep was not interrupted to a degree that would influence amenorrhea, then the hypothesis no longer holds.

Second, people need to know that there are many, many reasons babies wake at night and require feedings.  The fact that Dr. Haig opted to ignore all the others is disconcerting.  If a parent believes there is one, and only one, evolutionary reason for an infant waking, then we have done parents a disservice in disseminating information about infant biology and evolution.  Parents deserve to know the many things we have learned and to make their decisions based on all the information we can provide.  Similarly, even if this is one of the reasons babies may wake, it does not mean it is inherently the only one.  In fact, I would bet that babies wake for myriad reasons all at the same time.   As such, parents should be aware of all possibilities, how some may interact or overlap, and then look to their own child for the possible reasons (if they even care).

It is frustrating that I know I will have countless people defend sleep training to me under the guise that their babies are only doing it to “manipulate” them and we don’t live in a world where that’s necessary anymore.  It is frustrating that the media has a seemingly unstoppable urge to make splashes over anything that supports our modern parenting and decries the way we have historically cared for our children.  It is frustrating that an academic insinuates his hypothesis as fact when it comes to offering unsolicited advice on parenting.

The really frustrating part?  It truly is an interesting hypothesis worthy of discussion as it highlights a very intricate interplay between infant and maternal behaviour, but that seems to be lost in the fuss over sleep training and the ignorance of other possibilities.


[1] Haig D.  Troubled sleep: Night waking, breastfeeding and parent-offspring conflict.  Evolution, Medicine, and Public Health 2014; 1: 32-9.  (Read here.)

[2] Van Ginneken JK.  Prolonged breastfeeding as a birth spacing method.  Studies in Family Planning 1974; 5: 201-6.

[3] Thapa S, Short RV, Potts M.  Breast feeding, birth spacing and their effects on child survival.  Nature 1988; 335: 679-82.

[4] Diamond J.  The World Until Yesterday. New York, NY: Penguin Group, 2012.

[5] Hinde K.  Essential tensions in infant rearing.  Evolution, Medicine, and Public Health 2014; 1: 48-50.  (Read here.)