Physiology Fridays: Warm Milk

Okeedokee, today is a big one and it may look a little daunting, but it will be totally worth it in the end. I pinky swear.*

Also, I finished this up last night around midnight, so if there are more mistakes than usual, sorry. I'm just too lazy to go through it all right now and proofread (but not too lazy to type this explanation apparently). Anyway, on to the good stuff.

My son was having some troubles going down for naps a while back, so drawing on my own childhood, I turned to a little medicine my folks used to give me to help me sleep: milk (some of you were probably thinking: vodka! Shame on you. Straight vodka is not good for kids. White Russians on the other hand...).** And low and behold it worked. So now before naps and every night before bed, we give our 16 month old a glass of warm milk to help him fall asleep. Could there be some physiology behind it?

People have been using warm milk as a sleep aid for millions of years. If I had to guess, I'd say it correlates with the invention of the microwave. So, probably for the last 2 million years. Before that, they used to just give each other a little smack on the head with a club to help each other sleep (Note: I do not in any way condone domestic violence). But why warm milk? Is warm milk just like a little kid needs his teddy bear to sleep? Is it more psychological than anything else?

There are two theories currently in play right now. The first has to do with proteins, neurotransmitters, the pineal gland, and biochemistry. Big words. But it's not too complicated. So let's break it down.

Remember the pineal gland from back in high school? Probably not. I can barely remember what it does and I'm gonna be a doctor one day. The pineal gland is in charge of keeping time in the body. It does this through a hormone called melatonin (not to be confused with melanin which is in charge of skin color). Melatonin levels are high at night and low during the day. High levels tell your body it's time to sleep and low levels tell your body it's okay to be awake. So if we could artificially increase melatonin levels, we could trick the body into thinking it is sleepy time.

Now for some biochemistry. Melatonin is made from serotonin, which is really a cool hormone and will probably be a topic in the future, but for now, nobody cares about it. Except, that serotonin is made from a protein called tryptophan. So to summarize, tryptophan leads to serotonin which leads to melatonin which is how your body tells time.

When you warm up milk, it releases more tryptophan. So the theory goes that taking in excess tryptophan leads to increased melatonin. High melatonin then makes the person sleepy. While this works on the surface, and may play a small role, it does have some physiology issues. Tryptophan needs high amounts of insulin to cross into the brain. Insulin comes about when the body sees lots of sugar. Drinking milk shouldn't normally cause the high levels of insulin needed for a huge influx of tryptophan into the brain. So for milk to work, a bunch of carbs should be taken with the glass to help stimulate insulin and thus help melatonin cross into the brain. People don't usually load up on sugar right before going to bed though. So this probably isn't the major contributing factor to the sleepiness.

The second theory gets away from the big words for the most part and is the one I currently subscribe to.

Here's what is probably happening. Whole milk has a lot of fat in it. We give our son whole milk, the Romans gave their kids whole goat milk, ancient cave men used to give their kids milk straight from the stegosaurus. Whole milk is probably best for this to work. It sits in the stomach for a little while because of the fat and convinces the body that a good full meal was just eaten and so your body goes into "rest and digest" mode. When the body goes into the "rest and digest" mode, it shunts blood to the GI system and away from the periphery, which includes the brain. When the brain doesn't have blood, it doesn't have oxygen. Lack of oxygen tells the brain to go into sleep mode. And voila! Sleepy time.***

This is a temporary situation though. The body won't be fooled by the milk for long and will eventually return things to normal. So if you are going to drink milk, do it right before bed.

For those of you looking for a nice neat summary, we probably have a combination of the two at play - tryptophan increases the melatonin a little bit and contributes to sleepiness, but the large glass of milk telling hour body to send blood away from the brain is probably the main factor. Either way, milk is good for sleep and now you have some science to back it up.

*Legal note: pinky swears over the internet are non-binding and should not be considered as a real contract unless actual pinky-to-pinky contact was made in the physical world

**Please do not give your children alcohol ever, even if it sounded like a good idea when you read it on a blog.

***side bar - this explains why you feel sleepy right after lunch when you head back into work. Most people grab a McDonalds or some other fast food high in fat. Your body thinks it needs to supply your digestive system with blood and it borrows from the brain's supply. If you were to eat a small meal at lunch and another snack a couple of hours later, you would probably find you are less sleepy and have more energy to make it through the afternoon.