Coffee with milk can indeed make you sleepy due to the presence of tryptophan in milk.

Tryptophan is an amino acid that is commonly found in milk and other dairy products. It plays a crucial role in the production of seratonin, a neurotransmitter that helps regulate sleep and mood.

When you consume milk, tryptophan enters your bloodstream and eventually makes its way to your brain, where it is turned into serotonin.

As a result, your serotonin levels increase, promoting feelings of relaxation and sleepiness.

The combination of coffee and milk may have a sedating effect on your body. Coffee contains caffeine, a stimulant that is known to increase alertness and energy levels.

However, when combined with milk, the tryptophan content can counteract the stimulant effects of caffeine by promoting sleepiness. This can be particularly noticeable if you consume coffee with milk later in the day or evening, when your body is naturally winding down.

And when milk is warmed, for instance in a hot coffee vs. a cold coffee, the sleepiness can be intensified.

One may argue the amount of tryptophan is not aignificant enough to make a noticeable impact on your sleepiness levels.

While it is true that tryptophan levels in milk may not be as high as in certain foods like turkey, for example, it is important to consider that tryptophan is one of several factors that can influence sleepiness.

Additionally, individual sensitivities to tryptophan can vary, and some individuals may be more susceptible to its sleep-inducing effects.

Furthermore, the effects of combining coffee with milk may further be intensified when accompanies by other lifestyle factors.

For instance, if you are already sleep deprived or experiencing high levels of stress, the sedating effects of tryptophan in milk can be even more pronounced.

It is also wort noting that the overall composition of a meal or snack, combined with the presence of tryptophan in milk, can influence its impact on sleepiness.

Consuming a heavy meal or consuming large amounts of carbohydrates alongside coffee with milk may enhance the sedating effects.

Overall, the tryptophan content in milk can indeed make you sleepy when consumed with coffee.

The combination of stimulant caffeine and serotonin- promoting tryptophan can lead to a feeling of relaxation and sleepiness. 

While the exact impact of tryptophan in milk may vary depending on individual factors, it is important to considerthat its effects can be augmented due to other lifestyle and dietary factors.

Therefore, if you find yourself struggling to stay awake after enjoying a cup of coffee with milk, it may be worth considering the influence of tryptophan on your sleepiness levels.

Maybe next time try drinking it black- it may just be the answer!