Tryptophan

Tryptophan can be excited with ultraviolet radiation and it emits in the 300 to 450 nanometer wavelength range. The emission spectrum of tryptophan is very sensitive to the environment surrounding it. A Trp sitting on the surface of the protein generally has a low quantum yield and a red shifted emission (around 350 nanometers). A buried tryptophan, on the other hand, can have a variety of spectral characteristics depending on what other amino acids are adjacent to it in the structure. This sensitivity makes it a useful probe for studying changes in protein structure. Due to its significant permanent dipole moment, it can be used to monitor the local electrostatics in that part of the protein. The only other residues that fluoresce are Tyrosine and Phenylalanine although the have much lower quantum yields

Additionally, because of its aromaticity and mixed hydrophobicity and polarity (depending on how it is categorized), it can participate in a number of contacts inside and outside the protein. Tryptophan is often involved in stabilizing tertiary structure because of its large size. It is one of the most highly conserved residues evolutionarily (Dahyhoff, 1978) because it occupies so much volume inside a protein. It stacks with other hydrophobic residues, acts as a quadropole acceptor to cations and can even donate and accept hydrogen bonds.

See also: why does Tryptophan make you drowsy?

structural formula of tryptophan:

                         O        H
                          \\     /
     H        H   H   H    C -- O
      \      /     \ /    /
       C == C       C -- C -- N -- H
      /      \     /      \    \
H -- C        C - C        H    H
      \\     //   \\
       C -- C       C       tryptophan
      /       \   /  \      C11H11N2O2
     H          N     H
                |
                H

Of course, it's probably not that simple. In some mammals studied, melatonin levels did not increase until significantly into the dark period. Also, melatonin does not alter human sleep patterns unless ingested during certain times of day. This might rule out a simple "melatonin->sleepiness" trigger.

Sources:
Reppert and Weaver (1995) Cell 83: 1059-1062.
Ribelayga et al. (2000) Am. J. Physiol. Regulatory Comp. Physiol. 278: R1339-R1345.