It’s daylight science time again

It’s that time of year, when crocuses bloom, the lawn starts to need mowing, and most Americans lose an hour’s sleep setting their clocks ahead. (Remember? Spring forward, fall back.) So here are answers to your questions about the time switch — and about sleep.

Most Americans move their clocks ahead for daylight-saving time in the wee hours of the second Sunday in March. The day of the big switch used to be the first Sunday of April, but in 2005, Congress revised the rule as an energy-saving measure.

What's the rationale behind the switchover? As the year progresses toward the June solstice, the Northern Hemisphere gets longer periods of sunlight. Timekeepers came up with daylight-saving time — or summer time, as it’s known in other parts of the world — to shift some of that extra sun time from the early morning (when timekeepers need their shut-eye) to the evening (when they play softball).

The idea is that having the extra evening sunlight will cut down on the demand for lighting, and hence cut down on electricity consumption — and that few people will miss having it a little darker at, say, 6 o'clock in the morning. At least that's how the theory goes.

Not everybody goes along with the plan, as folks in places like Hawaii and most of Arizona know quite well. Each state or country comes up with its own schedule for the switch: Most European countries don't switch to summer time until the last weekend in March. And yes, some countries in the Southern Hemisphere are moving their clocks back an hour at this time of year.

If you’re in the “spring forward” mode, don’t lose any sleep over the hour you’re losing. Aside from leaving you a bit groggy, it won’t have much effect on your health. But Dr. Rosalind Cartwright, sleep expert at Rush Medical Center in Chicago, says that if you lose too much sleep, even a couple of hours for just two or three days, your immune system suffers, and you’re more susceptible to colds and viral infections.

She also explains that if you get to sleep too late, or up too early, your body will make sure it gets the deep sleep it needs for rest. What you lose is the light sleep during which you dream, which is important for mood. Which explains why you might feel groggy and grumpy after we “spring forward” to daylight-saving time.

When we wake up, why do we have crust in our eyes? - S.S.
“Eye crusts” are the leftover protein and fat from tears that have dried up. Tears have three components:

• Salty water, which comes from the tear gland behind the upper outer corner of our eye.
• Protein, secreted by the conjuctiva, which is the clear film that covers the eye.
• And fat, which comes from ducts in the eyelids.

Tears do lots of jobs. They clean the eye. They fill in tiny imperfections in the surface of the cornea, which needs to be perfectly smooth for maximal vision. They also deliver nutrients to the cornea, which has to be clear to let light through to the iris, and therefore has no blood vessels to deliver a “food supply.” And of course tears flow at times of emotion, when the tear glands power up and produce more salt water.

At night, with our eyes closed and protected, we don’t display emotion, and with our eyes closed we don’t get dirt in our eyes. So we don’t make tears. Small amounts of the mixture already on the surface of the eye seep out, but without a fresh supply of liquid from the tear gland, the fat and protein dry up.

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