According to 2Spare.com:
Many people find that, at the beginning and end of the day, a small lump of fluff has appeared in the navel cavity. The reasons for this have been the subject of idle speculation for many years but in 2001, Dr. Karl Kruszelnicki of the University of Sydney, Australia undertook a systematic survey to determine the ins and outs of navel lint. His primary findings were as follows:
- Navel lint consists primarily of stray fibres from one's clothing, mixed with some dead skin cells and strands of body hair. The rotting cells can create unpleasant odors.
- Contrary to expectations, navel lint appears to migrate upwards from underwear rather than downwards from shirts or tops. The migration process is the result of the frictional drag of body hair on underwear, which drags stray fibres up into the navel.
- Women experience less navel lint because of their finer and shorter body hairs. Conversely, older men experience it more because of their coarser and more numerous hairs.
- Navel lint's characteristic blue-gray tint is likely the averaging of the colors of fibres present in clothing; the same color as clothes dryer lint.
- The existence of navel lint is entirely harmless, and requires no corrective action.
But I don’t buy this. I think it comes more from your shirt than from your underwear. I notice that sometimes I’ll be wearing a newer dark sweater or shirt and that evening I’ll find dark belly button lint to match…. and I’ve been wearing tightie-whities that day.