Science of Nostalgia 

http://www.nytimes.com/video/science/100000002325921/science-of-nostalgia-.html

this is a video explaining nostalgia on how is nostalgia is good for you and how it can be bad for you as well, it is explained by John Tierney who is a columnist for the science times.

For the last two centuries it has been classified as an illness form of psychosis and a form of melancholia.
Nostalgia in greek has two roots nests meaning return home and alga meaning longing. nostos (return) alga (pain)
Nostalgia is good for you because it counteracts loneliness, anxiety, from looking back on your past it makes you feel more optimistic about what the future is going to bring.

http://www.nytimes.com/2013/07/09/science/what-is-nostalgia-good-for-quite-a-bit-research-shows.html?pagewanted=all&_r=1&

Constantine Sedikides was having lunch with a clinical phycologist and he described unusual symptoms and the phycologist put it down as depression. Sedikides says that “Nostalgia made me feel that my life had roots and continuity. It made me feel good about myself and my relationships. It provided a texture to my life and gave me strength to move forward.”

Nostalgia has been shown to counteract loneliness, boredom and anxiety. It makes people more generous to strangers and more tolerant of outsiders. Couples feel closer and look happier when they’re sharing nostalgic memories. On cold days, or in cold rooms, people use nostalgia to literally feel warmer.

Dr Sedikides says "Nostalgia makes us a bit more human."

In 1688 a swiss doctor called Johannes Hofer described Nostalgia as a Neurological disease of essentially demonic cause. Hofer believed that the cause of Nostalgia in the swiss soldiers was damage to the soldiers’ ear drums and brain cells by the unremitting clanging of cowbells in the Alps. Hofer's prescribed cures with leeches, purging, opium and warm hypnotic emulsions.

In the 19th and 20th centuries nostalgia was variously classified as an “immigrant psychosis,” a form of “melancholia” and a “mentally repressive compulsive disorder” among other pathologies. But when Dr. Sedikides, Tim Wildschut and other psychologists at Southampton began studying nostalgia, they found it to be common around the world, including in children as young as 7 (who look back fondly on birthdays and vacations).

most people report experiencing nostalgia once a week and to up to 3-4 times a week. these people are reported to have negative memories and opt for loneliness. Researchers at Southampton tested Nostalgia with people reading disasters and taking personality tests which made them feel extremely lonely and depressed. the people depressed about the disaster victims or worried about being lonely became less depressed and less lonely.

“Nostalgic stories often start badly, with some kind of problem, but then they tend to end well, thanks to help from someone close to you, because on the whole the positive elements outnumber the negative elements ” Dr. Sedikides says. “So you end up with a stronger feeling of belonging and affiliation, and you become more generous toward others.” 

It is more common to  feel more nostalgic in a cold room rather than  a warm room. Dr. Wildschut says, that nostalgia might have had evolutionary value to our ancestors long before Odysseus. 

http://www.nytimes.com/interactive/2013/07/09/science/nostalgic-songs-and-reader-submissions.html?ref=science

this is a playlist, they say to induce nostalgia is to listen to music. In an experiment in the Netherlands, Ad J. J. M. Vingerhoets of Tilburg University and colleagues found that listening to songs made people feel not only nostalgic but also warmer physically.

“Suite: Judy Blue Eyes,” by Stephen Stills: “Don’t let the past remind us of what we are not now.” This sense of loss and dislocation has repeatedly been linked to both physical and mental ills.

The feeling of Discontinuity isn't a symptom of Nostalgia, in fact people who nostalgia more frequently tend to have a healthier sense of self continuity.

 Saudade is a Portuguese and Galician word that has no direct translation in English. It describes a deep emotional state of nostalgic or deeply melancholic longing for an absent something or someone that one loves. Moreover, it often carries a repressed knowledge that the object of longing will never return. A stronger form of saudade may be felt towards people and things whose whereabouts are unknown, such as a lost lover, or a family member who has gone missing. 
http://en.wikipedia.org/wiki/Saudade




Nostalgia past present and future by Constantine sedikides, tim wildschut 

http://www.wildschut.me/Tim_Wildschut/home_files/Sedikides,%20Wildschut,%20Arndt,%20%26%20Routledge,%202008,%20CDir.pdf 

Symptoms included anxiety, sadness, and insomnia. By the mid-20th century, psychodynamic ap- proaches considered nostalgia a subconscious desire to return to an earlier life stage, and it was labeled as a repressive com- pulsive disorder. Soon thereafter, nostalgia was downgraded to a variant of depression, marked by loss and grief, though still equated with homesickness.

Nostalgia has been conceptualized variously as a negative, ambivalent, or positive emotion (Sedikides et al., 2004). These conceptualizations were put to test. In a study by Wildschut, Stephan, Sedikides, Routledge, and Arndt (2008), British and American undergraduates wrote narratives about a ‘‘nostalgic event’’ (vs. an ‘‘ordinary event’’) in their lives and reflected briefly upon the event and how it made them feel. Content analysis revealed that the simultaneous expression of happiness and sadness was more common in narratives of nostalgic events than in narratives of ordinary events. Also in Wildschut et al., British undergraduates wrote about a nostalgic (vs. ordinary vs. simply positive) event in their lives and then rated their happi- ness and sadness. Although the recollection of ordinary and positive events rarely gave rise to both happiness and sadness, such coactivation occurred much more frequently following the recollection of a nostalgic event. Yet, nostalgic events featured more frequent expressions of happiness than of sadness and induced higher levels of happiness than of sadness.
Wildschut et al. (2006) obtained additional evidence that nostalgia is mostly a positively toned emotion: The narratives included far more expressions of positive than negative affect. At the same time, though, there was evidence of bittersweetness. Many narratives contained descriptions of disappointments and losses, and some touched on such issues as separation and even the death of loved ones. Nevertheless, positive and negative elements were often juxtaposed to create redemption, a narrative pattern that progresses from a negative or undesirable state (e.g., suffering, pain, exclusion) to a positive or desirable state.

Wildschut et al. (2006) asked participants to describe when they become nostalgic. The most frequently reported trigger was negative affect (‘‘I think of nostalgic experiences when I am sad as they often make me feel better’’), and, within this category, loneliness was the most frequently reported discrete affective state (‘‘If I ever feel lonely or sad I tend to think of my friends or family who I haven’t seen in a long time’’). Given these initial reports, Wildschut et al. proceeded to test whether indeed negative mood and loneliness qualify as nostalgia triggers.