What the research describes about daytime energy

That afternoon dip isn't a lack of willpower. Your alertness runs on two systems at once: pressure that builds the longer you're awake, and a daily body clock with its own shape.

In most adults, those two pull against each other in the early afternoon. That's why the dip shows up at roughly the same time, lunch or no lunch.

What light does

Light is the main thing that sets your body clock, and research describes morning light as the strongest signal of all.

Here's the catch: indoor light is one to two orders of magnitude dimmer than an overcast morning outside. So studies of office workers often describe people getting almost no useful clock signal before noon.

Caffeine: what does half-life mean for you?

In most adults, caffeine has a half-life of roughly four to six hours. So half of an afternoon coffee is still in you at bedtime.

The research point isn't that caffeine stops sleep. It's that it cuts deep, slow-wave sleep while your time in bed stays the same. You sleep the same hours, wake less rested, and reach for more caffeine the next day.

Do meals cause the dip?

A little. Studies describe a post-meal dip that grows with bigger, more carbohydrate-heavy meals.

But the effect is smaller than the body-clock one. That's why the research tends to find that when you eat matters more than what you eat.

When should tiredness worry you?

Fatigue is one of the vaguest symptoms in medicine. It shows up in anaemia, thyroid disorders, sleep apnoea, depression and a long list of other conditions, several of them easy to test for and treat.

Reshaping your schedule and light makes sense when that's the cause. It's no substitute for a proper check when the tiredness is new, severe, or comes with anything else.