Although the diagram at right appears a little complicated it really represents the human body's 24 hour biological cycle which in medical terms is referred to as the "Circadian Rhythm". The body has an internal clock mechanism centered in the brain which governs such things as appetite and sleep. The schedule is set in part by external stimulus to the body such as the onset of daylight or darkness.

Sleep Masks work by covering the eyes and causing a biological and chemical change to occur in the body. When covered, light receptors in the eyes signal to the body the onset of darkness and therefore a change from the production of serotonin (the chemical of daytime) to melatonin (the chemical of night). In essence, Sleep Masks fool the body's internal clock to produce the chemicals that prepare it for sleep.

Light Blocking

Obviously the most important consideration is that the sleep mask mask blocks the light. The mask should be well shaped and designed to prevent light from leaking in from around the sides and top, but particularly you need to pay attention to the nose area. Many quality sleep masks take care of light leaks around the nose by sewing in a special piece of fabric that can be adjusted to fill in the space. Manufacturers of quality masks will also sew in a special blackout fabric that prevents light from penetrating directly through the front of the mask. Without full light blockage a sleep mask will not be able fool the brain into manufacture of the chemicals of sleep.

Comfort

The sleep mask needs to be comfortable to wear. If a mask is not comfortable, the wearer spends an undue amount of time trying to escape the discomfort rather than drifting peacefully into slumber. Generally the item that causes most grief for people is the strap that holds the mask to the wearer's head. Most of the time these are some form of elastic which can be fastened together with buckles or Velcro closures. The fasteners are often bulky and positioned right in the back of the head which contacts the pillow. Adjusting the strap so that it lies further towards the nape of the neck or slightly above the area of contact with the pillow will often take care of the issue however, some sleep mask manufacturers have managed to strategically offset the fastener so that it no longer poses a problem. For Velcro fasteners, arranging the position of hook and eye can often prevent people's hair from being caught as well.

One of my favorite mask manufacturers of quality masks is Dream Essentials®. They have come up with a unique solution to creating a truly comfortable head strap with their Comfort Snug™ Band. This is a very thin stretchy Lycra type strap that feels almost like you are wearing no strap at all, very comfortable, very clever and a unique solution to the major comfort issue with sleep masks today. Check out their Dreamer Sleep Mask

Eye Pressure

Another comfort issue is the amount of contact that the sleep mask exerts on the face and particularly the eyes. Some people find the sensation of fabric on the eyes to be comforting and others a discomfort. When traveling, women particularly prefer sleep masks that have hollow spaces for the eyes so as to not smudge their eye makeup.

As a side note here: One characteristic of drifting into a deep restorative sleep is the onset of REM (rapid eye movement). Some people believe that if a sleep mask touches the eyes and prevents them from movement, that the sleep will not be as refreshing (at this stage there does not appear to be any hard evidence to support this theory)

Eye Pressure or no eye pressure, really boils down to personal preference.

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Sleep Masks come in all sorts of shapes, sizes, thicknesses, materials and purposes. From the thin throw-away nylon masks that are often given away by airlines to the huge almost space helmet type of sleep contraptions that incorporate pillows, sound muffling devices and light and music displays. For most people, it is a very unusual sensation to have anything placed upon their heads and often takes some getting used to. When choosing a sleep mask, consideration must be given to the bulkiness of the mask and whether you would be comfortable wearing it all night.

Appearance

Although the look of the mask should take a backseat to such considerations as comfort and functionality, it is often the primary motivation in a person's buying decision. This is not unusual in circumstances in which the person is buying from a picture and does not have the ability to try the mask on before ordering (such as purchases made from mail-order catalogs or the internet).
Fortunately some sleep mask manufacturers make masks that are both very functional and stylish at the same time. A person can find a sleep mask that will match their bedroom decor or Armani suit.

Sleep masks have been around in some form or another since humans needed to sleep. It was only very recently however that manufacturers put some thought into improving the standard sleep mask design and creating mass market appeal.

Choosing a sleep mask is definitely not rocket science, but there are some things that need to be considered. Hopefully this Lens will have you on the way to making some informed buying decisions and obtaining the sleep you need.