Have you ever been bombarded with ads for an item immediately after you purchased it? I bet that struck you as immeasurably stupid. If you wanted two mattresses, or two pope hats, or two chainsaws, you'd have ordered two.

So what's the best time to advertise? Slightly before the time you'll want the item. More precisely, the best time is when other factors are also aligned: you have the money, you're about to get in the car, you're already shopping, you're engaged in a related activity, and so on, but let's focus on timing for now.

Slightly before means a prediction is necessary. When will you want ice cream next? Or a head of lettuce, a keyboard, or a car?

First, we have to recognize that much of what we do in life is repetitive and rhythmic. Everything we buy is either consumed or eventually needs to be replaced. Every item we own has a maintenance overhead attached to it, with the maintenance tasks also repetitive and rhythmic.

If someone knows your personal rhythm of buying lettuce or cleaning your home, they can make very reasonable predictions about when to advertise their grocery store, cleaning supplies, or cleaning service.

Let's say you bought a head of lettuce two weeks ago, a week ago, and today. That works out to once per week. It means that today is a very bad time to send you an advertisement for lettuce. Tomorrow is a bad time, as is the next day, and the next. But the time to advertise gets better and better until almost a week past your last purchase.

You probably own a vehicle so you need regular fuel fill-ups, oil changes, tire replacements, and other maintenance. When do you need your next oil change? Roughly the average interval between your last few oil changes. If you change the oil every year, then when it's almost a year since your last oil change it's a high probability that it's already on your mind. You'll actually appreciate a well-timed advertisement.

Generally, your openness to suggestion for any specific purchase depends on where in its rhythmic cycle you are. Think of a sine wave where the troughs represent the period of time immediately following your purchase and whose period between peaks represents the typical time between your purchases.

Is the wave low? That's a bad time to advertise: you are closed to suggestion for that item. Is the wave high? You are open to suggestion for that item. That's a great time to advertise.

All it takes to create such a wave is a series of timestamps of when you made your purchases, one series for each type of purchase. Then, whoever possesses your waves can predict with good accuracy, sometimes well into the future, when you'll be open to suggestion for specific items.

I suppose the question is, if well-timed advertisements are as simple as this, why do we see so many poorly timed and poorly targeted ads? It seems we have an opportunity to do much better, and dramatically improve the Internet experience for everyone.