Why do limbs fall asleep

[Christine]

You know how it goes. You sit too long with your legs crossed or lie with your arm under your head, and when you go to move the limb, it’s tingling with a “pins and needles” sensation. But why?
We’ve got nerves running through our bodies that act as lines of communication between the brain and the other body parts, transmitting commands from the brain and relaying sensory information back to it for processing. What’s happening with a sleeping limb is that your nerves are going a little haywire because prolonged pressure has actually cut off communication between that limb and the brain. (The tingling sensation is technically called paresthesia)
Pressure puts the squeeze on nerve pathways and blood vessels, so the nerves can’t transmit signals properly, and the blood vessels can’t bring oxygen and nutrients to the nerves. The cut-off interferes with the normal flow of information between the limb and the brain and the signals going back and forth get jumbled. Some nerve cells stop sending info entirely, while others send impulses erratically.
The problem is compounded by the fact that our nerves are pretty specialized and different kinds of nerves and sensory receptors receive different stimuli and transmit different information (last year, we talked about another bodily oddity caused by this). When the various signals get scrambled and aren’t transmitted normally, the brain starts to misinterpret the info it’s getting and generates an array of sensations, like warmth, numbness and that tingling feeling.
When a limb fallsasleep, we usually try to “wake it up” and change positions. Blood flows back to the limb, giving a little boost to the misfiring nerves and making the tingling seem worse, but eventually the nerve signals begin to flow properly again. The pins and needles sensation is annoying for a few minutes, but it’s a nice little prompt for us to relieve the pressure on a limb before serious nerve damage occurs.

[Christine]

You already know you'll lose an hour of precious sleep when the clocks shift forward into daylight saving time early Sunday morning.





But you may not know that you could lose a lot more if you don't take steps to ease the adjustment, experts said.

"There are more accidents on the road within two days of daylight savings," said Shelby Harris, director of behavioral sleep medicine at Montefiore Medical Center's Sleep-Wake Disorders Center in New York City. "If there are that many accidents, think about how many other things are affected."

Indeed, research released last fall in the New England Journal of Medicine revealed a jump in the number of heart attacks following both semi-annual time changes, although it was more pronounced in the spring.

According to Dr. Aparajitha Verma, medical director of the Sleep Disorders Center at The Methodist Hospital System in Houston, normal circadian rhythms (your "body clock") typically take 24 to 48 hours to readjust to time changes.

The good news is that a few simple steps can help smooth the transition. Take this advice from the experts:

• Move your bedtime up gradually by about 15 minutes every day for three days or so before the change. But don't get excited by the prospect of more sleep -- in order to fully adjust, you'll also need to wake up 15 minutes earlier. "You're basically trying to get yourself to bed a little bit earlier than you would ordinarily to get you in sync with the time change," said Dr. Nicholas Rummo, director of the Center for Sleep Medicine at Northern Westchester Hospital Center in Mount Kisco, N.Y.
• Make sure you take into account your body's other rhythms as well. Eat dinner an hour earlier and don't nap in the afternoon or evenings just before the time change, Harris advised.
• Expose yourself to bright light first thing in the morning, either by standing close to a window or even going outside for a few moments. "That tends to move the internal rhythm a little bit forward," Rummo explained. "Daytime exercise in good light has the same effect."
• Avoid caffeine and alcohol for at least six hours before going to bed, Verma said.
• Adjust your clocks earlier in the day on Saturday, not as you're going to bed. "That way, you're going by the new time earlier in the day," Harris said.
• Try to keep a light schedule for a day or two after the change, limit strenuous activity, and avoid driving if you can (think public transportation).

And don't forget that the time change can be even more of a disruption for people who have underlying sleep disorders. These people should take extra special care.

"People with underlying sleep disorders -- for example, sleep apnea -- may need to be treated in conjunction with the time change," Verma said.

An estimated 50 million to 70 million Americans have some kind of sleep problem, according to the Institute of Medicine.