Summary: We produce our own sleepiness hormone, but only in darkness
Action needed: Avoid bright lights in the evening; try to see natural light in the morning
It was a rainy morning here in Singapore, which makes me glad to have a source of 480 nm light. Let’s see why..
Slaves to the clock
The existence of daily (“circadian”) rhythms is well-known. There are documented downsides to running out of sync with the environment for some species: later flowering and reduced viability for plants, and small (< 20%) lifespan reductions for hamsters . Amazingly, this clock system mechanism is present in individual cells, but they must be synchronized via input from a master clock in the brain . It is therefore important to keep the master clock in sync with the environment.
Besides interference from chronic alcohol intake , the main external influence is the light-dark cycle. A hormone called melatonin is heavily involved; it attenuates the wake-promoting signal of the circadian pacemaker , thus leading to sleepiness. Its synthesis is dramatically affected by light exposure to the eyes; levels are very low during the day . They peak around 0400 , when wakefulness and alertness are minimal (useful for raids, hence the term “KGB hour”). Light exposure causes a phase shift in the rhythm whose direction depends on the current perceived time .
Unfortunately, we are constantly delaying our internal clock by exposure to bright light in the evenings . This is the source of the mistaken belief that our preferred day length is 25 hours – test subjects were allowed to use electric lighting before sleep . In fact, our timer periods are remarkably close to 24 hours, with an error of only 0.7%. 
More exposure to light causes corresponding melatonin inhibition; ordinary fluorescent lamps are sufficient, and the maximum is reached after an hour of bright light  (easily exceeded by computer monitors). To avoid desyncing our clocks and affecting sleep, we should dim lights in the late evening, avoid computer and TV screens, and possibly even wear sunglasses.
This reminds me of Edison’s poster with bold claims of being “in no way harmful to health”. Something to reflect upon: how many of our current practices will viewed as ignorant/hubristic/naïve by future generations?
Conversely, bright light in the morning is helpful for increasing wakefulness, for example by simply gazing into the dawn sky . If natural sunlight is lacking, how can it be emulated? Maximum responsiveness for the relevant photoreceptors (which interestingly are separate from the well-known rods and cones) is reported at 480 nm [8,10], which corresponds to blue light. In fact, there is a broad peak between 450 and 500 nm . However, other photopigments with absorption maxima closer to 420 nm also play a part . Lights with a bluish tinge will therefore be more effective than green.
We have seen that light affects melatonin in both undesirable and potentially helpful ways. There is also the option of melatonin supplements. When taken close to the target bedtime after long flights, it does indeed decrease symptoms of jet lag , but probably only because it increases sleepiness . However, beware: “Oral doses (1 to 5 mg) [..] result in serum melatonin concentrations that are 10 to 100 times higher than the usual night time peak”. To maintain concentrations within the normal range, the dose must be much lower – 0.1 to 0.3 mg .
Rounding out the discussion, there exist other clocks, for example in the liver , that are not directly synchronized to the master clock . A hormone called leptin acts as a feedback signal from adipose tissue (fat deposits) to the brain that too much energy has been consumed . The leptin rhythm can be shifted independently of the circadian rhythm by simply altering meal times . To ensure the feedback works as desired, we should maintain regular meal times and avoid midnight snacks.