LED lights behave strangely, why?
Here is the forward conduction characteristic of a typical LED: -



At 2V forward voltage the LED consumes 20mA and is quite bright. At 1.8V forward voltage the current has fallen to 5mA and the LED will be somewhat dimmer than the 20mA scenario. At 1.6V the LED is taking almost zero current and below this voltage it is doubtful that it illuminates at all.

If you have 10 similar LEDs in series, to obtain a forward current of 20mA requires a forward voltage of 20V. 

If you are happy with a forward current of 5mA then 18 volts is all that is needed. And, if your power supply is (say) 20V, a series resistor is needed that drops 2 volts at 5mA therefore its resistance will be about \$\dfrac{2V}{5mA}\$ = 400 ohms.

If you have a limited supply voltage, you can wire the LEDs in parallel banks or use a boost converter to lift your 5V to about 20 volts. The CAT4238 device from Motorola can do this: -