Sayaka joined DREAM after the original member Mai Matsumuro left the group and AVEX held auditions to replace her but ended up joining in 6 new members (and making of DREAM a 8 member unit). She's stayed on the group ever since (2002) and survived the period after AVEX told them to disband, performing on the streets with her fellow DREAM members as a 6 piece unit until LDH decided to sign them.
Last year DREAM lost their last original member, Kana Tachibana, who's recently returned into the stages accompanying Yuu Shirota on his tour and who's already working on her solo career. Ever since, Sayaka started thinking about her own dream, and now she decided to finally graduate to pursue a solo career (still not revealed if under LDH just like Kana did).
DREAM doesn't seem to have re-established the success they had once, not even after landing some media attention because of being part of E-Girls, such as their TV show E-Girlsが！モンクの叫び (E-Girls ga! Monku no Sakebi), and even their last single, Dream Girls, had been pushed back again and again and didn't get enough sells after being released, though E-Girls seems to have better luck with that.
Here's the message Sayaka has post at their official site:
Thank you so much for always supporting DREAM.
I, Sayaka, have been allowed to work with DREAM since 2002.
The DREAM that went from the members joining in (she refers to her entrance and the current 5 members plus Ai Risa who left on 2004) to the graduation of some (she particulary refers to Yu Hasebe's graduation, the second original member to leave) and the crisis we suffered after being disbanded (by AVEX) but were introduced to the Great company of LDH, starting with HIRO from EXILE who is the president of the company, and got a lot of support from the staff, getting a chance to dream once again.
Last year our leader Kana graduated, and we as 5 kept on chasing that dream.
I also want to challenge myself to a new dream, so I came to the conclusion I shall graduate from DREAM.
For this new destination, I still want to continue with singing.
In order to do this I need to grow on my own.
To get a future of my own I've decided to leave DREAM and dare to pursue my own dream.
I'll keep working as hard as I've been doing on DREAM ever since I stepped into this world.
To continue as a soloist is something I haven't experienced yet and makes me anxious but I will work hard to get the determination to step into this new enviroment .
From now on DREAM's Sayaka is over and I turn into the dream of being Sayaka Yamamoto with all my efforts, wishing the best to DREAM members to continue on pursuing their dream.
To everyone who supported DREAM and Sayaka,
Thank you so much.
I will work with all my heart for everyone.
Thank you for everything you've done until now.
Sayaka from DREAM.
Sayaka has had previous solo works, releasing two mini-albums and a single back on 2005, Sekishun and Senkou Hanabi (mini-albums) and the single Botan Yuki no Furu-gai, and another mini-album on 2006, Tsukushi no Kotozute.
As for DREAM, the group has suffered so many changes ever since AVEX created them on 2000. The most thraumatic ones were when Mai Matsumuro, the principal lyricist of the group, decided to graduate leaving Kana and Yu who didn't want a replacement dealing with 6 new additions; when AVEX decided to change their name from DREAM to DRM, confusing their fans and the rest of the audience; when AVEX told them to disband, and cancelled any deals with the group and, last, when Kana, the one who had taken on the leadership of the group, left. Probably this new change will affect the group, but for now it seems it will remain as a 4 piece unit.
Source: Official Website