How to increase serotonin
On a temporary basis, you can consume tryptophan or 5-HTP (5-hydroxytryptophan). These are precursors to serotonin. Tryptophan is a two-step conversion process that is negative-feedback regulated. 5-HTP is a one-step conversion process that is not negative-feedback regulated.
If your serotonin is low because of chronic inflammation, tryptophan and 5-HTP supplementation might boomerang on you. Inflammation activates IDO (indoleamine dioxygenase), which catabolizes (burns up) both tryptophan and 5-HTP. This sabotages the serotonin increase to some extent, but it also causes the synthesis of two neurotoxic metabolites from tryptophan, and unknown catabolites from 5-HTP. This can cause a next-day malaise from taking tryptophan or 5-HTP.
There is a work-around for this kind of problem. You can pre-dissolve your dose of tryptophan or 5-HTP in water by adding predigested (hydrolyzed) collagen protein. The collagen peptides solublize the tryptophan, which would otherwise be minimally soluble, and 5-HTP, which is slightly water soluble already. This tryptophan cocktail, when drunk on an empty stomach, greatly speeds the absorption and utilization of tryptophan from the gut.
In addition, the atypical amino acid profile of collagen shifts the ratios of amino acids in the blood stream to lower those that compete against tryptophan for transport across the blood-brain barrier. There is little phenylalanine, tyrosine, methionine and histidine (large, neutral amino acids) in collagen protein, so the large neutral amino acid transporter is more available for transferring the tryptophan (and 5-HTP) into the brain from the blood stream. So more tryptophan and 5-HTP is converted into serotonin, and less is catabolized into neurotoxic organic acids.
This significantly potentiates the effect of tryptophan. From my own experiences, I'd estimate that the enhancement is roughly eight-fold. Instead of 1500-2000 mg of tryptophan, I get a better effect from only 200-250 mg dissolved in predigested collagen solution. I use two teaspoons of collagen peptides. This reduction of total tryptophan dosage also degreases the neurotoxic catabolites of tryptophan.
On a sustainable basis, there are other things to consider.
(1) Negative ions (superoxide anion) are involved in the conversion of tryptophan to 5-HTP. If you live in a positive-ion polluted environment, an increase in negative ion exposure could easily raise your serotonin.
(2) Reducing inflammation. Chronic infections, allergies (IgE mediated), delayed hypersensitivities (IgG, IgA and IgM mediated), leaky gut syndrome, heavy metal toxicity, maldigestion and an unbalanced microbiome can all raise inflammation. Lowering inflammation should lower IDO and increase serotonin sustainably.
(3) If hypometabolic, raising your metabolic rate. Low metabolic rate causes rationing of energy by many body systems, including (a) neurotransmitter synthesis, (b) antioxidant defense (the recycling of vitamin C and glutathione), (c) healing of tissues, particularly the intestine and bowel epithelial linings, (d) digestive enzyme production, and (e) hydrochloric acid production by the stomach. Most of the time, hypometabolism (hypothyroidism) is associated with adrenal compensation, which shifts the body and mind from a parasympathetic-dominant state (favoring relaxation, digestion, healing and sleep), into a sympathetic state (inhibiting digestion, healing and sleep, and promoting edginess and anxiety).
This answer is by no way complete, but has become a dissertation, so I'll stop at this point. But this is a very good question. Depression and sleep problems are endemic in our society for many of the above-described reasons. This happens to be one of the most common topics that comes up repeatedly at the monthly self-care meetups that I host here in Silicon Valley. Good luck on your further inquiries.