LEDs are most efficient at a current and light intensity that is much higher than what we want, so pulsing the LED is the solution. The frequency should be high enough to not be perceived as flickering (> 50Hz)

Ted's TritiLED project used a PIC microcontroller for this. Christoph's ultra low power LED uses an RTC module that combines crystal and RTC IC (the Micro-Crystal RV-3028-C7).

Both these are really overkill. They require loading either program code (for the PIC) or configuration values (for the RTC) into the respective flash memory or EEPROM. And while 1ppm accuracy in the RTC is nice, it's really not needed.

I say let's go back to the roots - two transistors are really all that's needed for an oscillator.

Dave Johnson at DiscoverCircuit.com proposes a circuit with NPN and PNP transistors connected a** to a** ... I mean emitter-to-emitter.

The 3nA variant uses 1Gohm resistors which are not something everyone has in their parts bin. Also a typical multimeter can't measure that, so troubleshooting is more difficult. The 30nA/100Mohm variant is more reasonable in that regard.

The original circuit targets a 1Hz rate, for a faster oscillation we decrease the capacitance values.

Replacing the 4.7Mohm resistor connecting Vcc to the output with a much lower 680kohm value means that in the idle state, the output level is much closer to the positive supply rail - much better for the next CMOS stage.