It's normal. not to worry.. for detailed information do direct online consultation

This is  longitudinal melanonychia. This  probably could be due melanocyte activation. No need to do anything about it.

The decolorization is usually in a stripe that starts at the bottom of your nail bed and continues to the top. It may be in one nail or several. These lines can be a natural occurrence. No matter what the cause may be, you should always have a doctor check any melanonychia. When a person has linear melanonychia, they may see dark stripes running down the nails. They may have colour variations that range from black to deep brown and grey. These lines will normally appear on 2 to 5 fingernails, but not necessarily all 10. The treatment options for melanonychia will depend on the primary cause. If the cause is benign, the individual may not need treatment. Some cases of melanonychia may fade after addressing the cause. Its usually self limiting. Consult a Dermatologist for the same.