Sandhyaa word has its origin from the word "Sandhi" which means "joining". So Sandhyaa can be of anything two when joined. There are several Sandhyaa --

(1) Sandhyaa Time
When day and night meet, i.e. in the morning at dawn and in the evening at dusk, is called Sandhyaa. Some people consider three Sandhyaas - they consider the noon time also as Sandhyaa, although by definition it does not seem so. They call Sandhyaa when two Prahar meet. Since there are 8 Prahar in a day, 3 hours each, 7 Sandhyaa times can be identified. But since people are awake only in day time, three Sandhyaa are common.

(2) Sandhyaa Worship
In Hindu religion Sandhyaa worship is considered a very necessary worship. This short worship is done at the time of dawn and dusk, when day meets the night, everyday. This worship is done at those times that is why it is called Sandhyaa worship. According to Ved Vyaas Jee Sandhyaaa is a prayer to God that we offer in the morning and in the evening. Yaagyavalkya Jee says - "We should perform this service thrice a day before the Sun sinks entirely below the horizon, before the Sun has entirely risen above the horizon, and at midday.

(3) Yug Sandhyaa or Sandhyaansh
There is another Sandhyaa, which comes before a Yug starts, and at the end of the Yug. This is normally of the same number of years (in hundreds) which are in Yug, for example, Sat Yug is of 4,000 Divine years, so its Sandhyaa is 400 Divine years. This is also called Sandhyaa - Yug Sandhyaa.

(4) Devee
Sandhyaa is a Devee also.

(1) Sandhyaa Time, and (2) Sandhyaa Poojaa
There is one thing which can be adopted, even along with other practices, and gives miraculous results to all. That is called Tri-Kaal Sandhyaa. It is an ancient, but scientific technique, given by our Rishi to awaken the moral, physical and spiritual self. Three times in 24 hours are called as Sandhya times when two Prahar meet. Ten minutes before and ten minutes after the sunrise, ten minutes before and after noon, and ten minutes before and after sunset are considered to be very auspicious and meditation, Jap or Praanaayaam done at this time is very helpful in all round growth of a person. Sandhyaa Vandan essentially is a Vaidik prayer. The scriptures (Taittireeya Aranyakaa, 2 Prapathak, 2nd Anuvaak) says that the Braahman who meditates on the rising and the setting Sun as the Supreme Being enjoys all happiness. The Vishnu Puraan says that who do not perform the Sandhyaa service, are sent to the dark Hell. Gobhil says in his Smriti that one who neglects the Sandhyaa becomes the Shoodra while living and a dog after the death. Yaagyavalkya Jee declares in his Smriti that he who omits the Sandhyaa in the absence of sufficient reason, such as illness, comes to ruin. Pulastya Jee lays down that the service should be gone through even in the times f pollution. Gautam Rishi (in II, 10-11) says that "And the morning and the evening devotions, Sandhyaa, must be performed outside the village. Silent he shall stand during the former, and sit during the latter, from the time when one light is still visible until the other light appears. (compare this to Aapstambh, I, 11, 30, 8). Raamaayan and Mahaabhaarat frequently make mentions of this service and we see that how Raam and Yudhishthir went sincerely through this ceremony, even in the times of emergency.

There is an Arthvaad on the subject in the Taittireeya Aranyakaa (2 Prapathak, 2nd Anuvaak) - "In former days, the evil spirits performed a severe penance. Prajaapati granted them a boon. They made this request that "the Sun should consent to fight with them." Prajaapti said to them, "Fight on." And so the evil spirits fight with the rising Sun till he sets, and these evil spirits are vanquished  by the water consecrated by the Gaayatree." Therefore it is that the wise Brahmvaadee in this world, face the East during the morning twilight and throw up watewr consecrated by Gaayatree. These waters become thunderbolt and hurl the evil spirits to the island called Arun where these evil spirits, called Mandeh, live. These Brahmvaadee then circumambulate around themselves and by doing this they scatter away their sins."

The Process of Sandhyaa Worship
One can wash his hands and feet, drink three sips of water and then do the Sandhyaa. If one is in office, then also it can be done - mentally. Praanaayaam done at these times is extremely helpful and enhances the mental faculties and physical strength. If you take out 15 minutes to meditate and do Praanaayaam at these three times in the day and lead a Saatwik life, you will have wonderful experiences and in just forty days you will find many positive changes in your health, mind and attitude towards life. Your memory will increase miraculously. You will be happier and more peaceful and become successful in life.

First comes Aachman (sipping water), a little water is taken in the right palm, just sufficient to moisten of a grain of a pulse and sip it three times. It is a purificatory act and is usually performed in all Hindu religious rites before commencing it. The palm is washed. Then the nine body parts (head, two eyes, two ears, two noses) are touched.

A Sandhyaa Vandan or Sandhyaa Poojaa to be performed daily may take the following steps:-
1. Aachaman
2. Ganapati Dhyaan
3. Praanaayaam
4. Sankalp - Sankalp Mantra is "Mam Upaattam Samast Durit Kshaya Dwaar Parameshwar Preetyarthe Praatah (or Saanya) Sandhyaa Mupaa Shishye)
5. Maarjan
6. Praashan
7. Punar-maarjan
8. Arghya Pradaan
9. Praayashchittaargya
10. Aikyaanu-sandhaan
11. Dev Tarpan
12. Jap Sankalp
13. Pranavayaas
14. Praanaayaam
15. Gaayatri Aavaahan
16. Gaayatri Jap
17. Gaayatri Upasthaan
18. Soorya Upasthaan
19. Samashti Abhivaadan
20. Digdevataa Vandan (Vandanaa of all direction' Devtaa)
21. Yam Vandanaa
22. Harihar Vandanaa
23. Soorya Naaraayana Vandanaa
24. Samarpan and
25. Rakshaa
Kar Nyaas and Ang Nyaas are the part of "Saangopaang Gaayatri Aavaahan".
Apart from chanting Rishi, Chhand, and Devtaa; Kar Nyaas and Ang Nyaas, Dhyaan and Panch Poojaa are also to be done.

(3) Yug Sandhyaa or Sandhyaansh
These are the periods of beginning and end of a Yug respectively. Since these periods also fall at the joining of two Yug, that is why it is also called Sandhyaa and Sandhyaansh. Normally they are of the same number of years in hundreds as the number of years of that Yug in thousands. For example, Sat Yug consists of 4,000 Divine years, so it will have 400 years of its Sandhyaa and 400 years of Sandhyaansh. In the same way Tretaa Yug consists of 3,000 Divine years, so it will have 300 years of Sandhyaa and 300 years of Sandhyaansh. One should take it like that for the rest of the two Yug also. Sandhyaa is before starting the Yug, and Sandhyaansh is after the Yug ends, thus Yug is the middle period of Sandhyaansh and Sandhyaa respectively.

Why to do Hom in Sandhyaa Times?
Whatever Hom is done in morning Sandhyaa, its effect, air purifying, continues up to evening Sandhyaa. and whatever Hom is done in evening Sandhyaa, its effect continues till next morning. That is why one should do Hom in both the Sandhyaa.